Achieving Stability and Lower-Limb Length in Total Hip ......limb length discrepancy. The...

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2010;92:2737-2752. J Bone Joint Surg Am.Keith R. Berend, Scott M. Sporer, Rafael J. Sierra, Andrew H. Glassman and Michael J. Morris

ArthroplastyAchieving Stability and Lower-Limb Length in Total Hip

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InstructionalCourse LecturesThe American Academy of Orthopaedic SurgeonsKENNETH A. EGOLEDITOR, VOL. 60

COMMITTEEKENNETH A. EGOLCHAIR

FREDERICK M. AZARMARY I. O’CONNORMARK PAGNANOPAUL TORNETTA III

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DEMPSEY S. SPRINGFIELDDEPUTY EDITOR OF THE JOURNAL OF BONE AND JOINT SURGERYFOR INSTRUCTIONAL COURSE LECTURES

Printed with permission of the American Academy ofOrthopaedic Surgeons. This article, as well as other lecturespresented at the Academy’s Annual Meeting, will be availablein February 2011 in Instructional Course Lectures, Volume 60.The complete volume can be ordered online at www.aaos.org,or by calling 800-626-6726 (8 A.M.-5 P.M., Central time).

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Achieving Stability and Lower-LimbLength in Total Hip Arthroplasty

By Keith R. Berend, MD, Scott M. Sporer, MD, Rafael J. Sierra, MD, Andrew H. Glassman, MD, MS, and Michael J. Morris, MD

An Instructional Course Lecture, American Academy of Orthopaedic Surgeons

Total hip arthroplasty is an exceptionallycost-effective and successful surgicalintervention1,2. Dislocation, infection,osteolysis, and limb length inequality areamong the most common complica-tions affecting the long-term successof total hip arthroplasty2-8. Instabilitywith dislocation is a complication thatis costly to the patient, surgeon, andhospital9. The surgeon is frequentlyfaced with the challenge of obtaininga stable hip at the cost of increasing thelength of the lower extremity10. ThisInstructional Course Lecture addressesthe common issues that surround theachievement of both stability and limblength equality with total hip arthro-plasty. We review the preoperative pa-tient education and factors associated

with stability and limb length, the effectand role of various surgical approaches,the surgical techniques, and the man-agement of instability with and withoutlimb length inequality.

InstabilityDislocation rates are reported to be0.3% to 10% after primary total hiparthroplasty and up to 28% after re-vision total hip arthroplasty. The in-cidence appears to be highest within thefirst year and rises at a rate of about 1%per five years to 7% at twenty-five yearspostoperatively11-21. A recent nationaldatabase study revealed that instability/dislocation was the most common di-agnosis resulting in revision total hiparthroplasty in the United States3. There

are patient-specific risk factors associ-ated with instability, including femalesex, increasing age, a diagnosis ofosteonecrosis or femoral neck fracture,obesity, a high preoperative range ofmotion, and comorbidities5,13,15,22-33.There are variables under the surgeon’sdirect control, including the surgicalapproach, component position andorientation, femoral head size, restora-tion of offset, preservation of soft-tissueintegrity, limb lengths, and prostheticimpingement. Surgeon experience isa variable, and the risk of instability isinversely related to the case volume ofthe operating surgeon5,13,15,30-32,34,35.

Preoperative EvaluationPostoperative limb length inequalityand hip instability are common causesof litigation36-38. A thorough preopera-tive discussion establishes realistic pa-tient expectations, and a hierarchy ofreconstruction goals should be out-lined: first, well-fixed acetabular andfemoral components; second, a dy-namically stable construct; and third,equalization of limb lengths. The pa-tient must understand and accept thatlengthening of the lower limb may be

Look for this and other related articles in Instructional Course Lectures,Volume 60, which will be published by the American Academy of Or-

thopaedic Surgeons in February 2011:

! ‘‘The Evolution and Modern Use of Metal-on-Metal Bearings in Total Hip

Arthroplasty,’’ by Mark Gonzalez, MD, MEng, Ryan Carr, MD, Sharon

Walton, MD, and William M. Mihalko, MD, PhD

Disclosure: The authors did not receive any outside funding or grants in support of their research for or preparation of this work. One ormore of the authors,or a member of his or her immediate family, received, in any one year, payments or other benefits or a commitment or agreement to provide such benefitsfrom a commercial entity in excess of $10,000 (Biomet and Zimmer) and less than $10,000 (Innomed).

J Bone Joint Surg Am. 2010;92:2737-52

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THE JOURNAL OF BONE & JOINT SURGERY d J B J S .ORG

VOLUME 92-A d NUMBER 16 d NOVEMBER 17, 2010ACHIEV ING STABIL ITY AND LOWER-LIMB

LENGTH IN TOTAL HIP ARTHROPLASTY

required in order to achieve the firsttwo goals.

A complete medical and surgicalhistory should be obtained. Previoussurgery on either extremity can createlimb length inequality that is not ap-preciated on a pelvic radiograph alone.Previous fracture, infection, physeal ar-rest, and various dysplasias may result inlimb shortening. Abnormalities of theaxial skeleton, such as prior spinal fusion,scoliosis, or neuromuscular disorders,or soft-tissue contractures associatedwith the hip or knee result in apparentlimb length discrepancy. The combi-nation of ‘‘true’’ and ‘‘apparent’’ limblengths contribute to the patient’s sub-jective perception of limb length in-equality (Figs. 1-A and 1-B)39.

Physical ExaminationObservation of the patient’s gait identifiespelvic obliquity, weak abductors, and

dependence on assistive devices. Themajor muscles around the hip (abduc-tors, adductors, and flexors) as well as theiliotibial band are assessed for contrac-tures. The levels of the iliac crests are com-pared with the patient standing (Fig. 2),and the thoracic and lumbar spine isassessed for coronal or sagittal deformity.

True limb length is determinedby measuring the actual length of theextremity clinically or radiographically.The apparent limb length is determinedby adding the effects of pelvic obliquityand soft-tissue contractures. Clinically,true limb length is measured from theanterior superior iliac spine to the me-dial malleolus (Fig. 3). Accurate identi-fication of the osseous and anatomiclandmarks can be difficult, especially inobese patients. A compensatory, flexiblescoliosis may develop in the presenceof a true limb length inequality. Theflexible deformities correct when a block

is placed under the shorter extremity orwhen the patient sits. A rigid coronalspinal deformity remains unchangedwith these maneuvers.

Radiographic AssessmentStanding anteroposterior pelvic, antero-posterior femoral, and lateral femoralradiographs should be obtained. Be-cause an arthritic hip frequently hasan external rotation deformity, theanteroposterior pelvic and femoralradiographs should be made with thefemur in 20! of internal rotation toavoid underestimation of femoral offset(Figs. 4-A and 4-B).

Preoperative radiographs providean estimation of true limb length in-equality. A line drawn between theinferior aspects of the obturator foram-ina, ischia, or radiographic ‘‘teardrops’’on a supine anteroposterior view of thepelvis is used as the pelvic reference. The

Fig. 1-A Fig. 1-B

Fig. 1-A There is a limb length discrepancy, with the right lower limb shorter than the left. Note the elevated heel of the right foot.Fig. 1-B Adduction contracture is present on the left, leading to the appearance of, but not true, limb length discrepancy.

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distance between this line and a fixedpoint on the femur (the lesser or greatertrochanter) can be compared with that ofthe contralateral hip. The differencebetween these two distances is the truelimb length inequality (Fig. 5). Thismethod is valid only if the limb lengthsare equal below the chosen referencepoint and the two lower limbs are held inthe same anatomic position.

Preoperative templating is essen-tial to minimize limb length inequality,restore offset, and therefore minimize thepossibility of instability (Fig. 6). First,the new center of rotation for the hip isdetermined by selecting the optimalposition of the acetabular component. Ingeneral, the inferomedial aspect of theacetabular component is placed in closeapproximation to the radiographic tear-

drop such that the inferiormost aspectof the acetabular implant template isaligned with the radiographic teardrop inthe vertical plane.

With femoral templating, theexaminer should determine:

1. Prosthetic size—i.e., the fit andfill of the femur needed to achieve axialand rotational stability.

2. Component offset. Extended-offset implants or a lateralized acetabu-lar liner may be required to restoreoffset.

3. Limb length. Limb lengthening(or, rarely, shortening) is planned on thebasis of the preoperative radiographicevaluation as well as the clinical assess-ment of apparent limb length inequality.

Not all patients with a true limblength inequality require lengthening.

Patients with a fixed adduction con-tracture or a pelvic obliquity may feelthat the limb is excessively long if thetrue limb length is restored. A commonreason for dislocation is the failure toadequately restore offset, which is thedistance between the center of hiprotation and the center of the femoralcanal40. Technically, templating can beperformed on the contralateral, normalhip and changes in limb length or offsetcan be extrapolated to the hip that is tobe operated on. Subsequently, femoralhead-neck length and implant offset canbe anticipated. Alternatively, templatingof the hip that is to be operated on canallow immediate recognition of howmuch length or offset will be changed byanatomic placement of components,compared with the nonoperative side.

Patient ExpectationsPreoperative discussions about limblength inequality and the possibility ofhip dislocation are critical and should setrealistic goals and reiterate the hierarchyof surgical priorities36. Patients mustbe aware that in some situations thelower limbmust be lengthened to achievecomponent stability. Additionally, pa-tients should be told that their lowerlimb will feel long immediately after thesurgery and that this is a normal phys-iologic response following hip replace-ment. Patients who have a sense that thelower limb is longer preoperatively butactually have normal limb lengths, orthose with a shortened extremity but theperception of equal limb lengths, areparticularly at risk for perceiving thatthey have a discrepancy after surgeryand should be appropriately warnedpreoperatively41.

Advantages and Disadvantages ofSurgical Approaches in Terms ofLimb Length and StabilityAnterior ApproachesThe true anterior approaches exposethe hip through the interval betweenthe sartorius and tensor fascia femorismuscles, with several variations. Theclassic approach is the Smith-Petersenapproach with either preservation ordetachment of the direct head of therectus femoris tendon. A variation of

Fig. 2

Standing evaluation of clinical limb length is performed by mea-suring the pelvic obliquity and limb length difference. The exam-iner’s hands palpate the superior iliac crests, and blocks areadded under the short lower limb until the pelvis is level. The blockheight needed to level the pelvis is the limb length difference.

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this approach, the Hueter approach (afascial incision over the tensor fasciafemoris), has gained interest because ofits theoretic ability to provide pro-tection to the lateral femoral cutaneousnerve, which is at risk with the classicSmith-Petersen approach42-44.

Limb length: A major advantageof the direct anterior approach is theability to directly measure limb lengthsbecause the patient is in the supineposition and the true limb length canbe measured at the ankle or heel. Anintraoperative supine radiograph orfluoroscopy is helpful for the measure-ment of limb lengths and componentposition. Studies have shown an averagemean limb length discrepancy of 3.9 mmwith use of this approach43,44. This smallamount of lengthening is well toleratedand accepted by the patient, making thisapproach one of the most accurate interms of limb length reconstruction.

Stability: The direct anterior ex-posure is a true internervous planebetween the sartorius (femoral nerve)and tensor fascia femoris (superiorgluteal nerve). This approach minimizessoft-tissue damage about the hip andpreserves the major abductor attach-ment. Only the anterior aspect of thecapsule is excised. Advocates point outthat no muscle detachment is neces-sary in order to deliver the femuranteriorly. The dislocation rate aftera single-incision anterior approachranges from 0.6% to 1.3%42-44.

Disadvantages: The approach istechnically demanding and may or maynot require the use of a specializedfracture table. There is a steep learningcurve associated with the procedure42,45.The lateral femoral cutaneous nerve isalways retracted, and the risk of injury tothis nerve should be discussed with thepatient preoperatively.

Two-Incision TechniqueThe two-incision technique was de-scribed by Light and Keggi46 and waspopularized by Berger47,48. It is basicallyan anterior Smith-Petersen approachwith an additional posterior smallerincision for placement of the femoralcomponent.

Limb length: The advantages ofthis approach are similar to those of thedirect anterior approach.

Stability: Excessive femoral ante-version is a risk because it is difficult tomaintain anatomic version while in-serting the femoral component throughthe small posterior incision. The re-ported dislocation rate after this pro-cedure is relatively low (1.0%)47-50.

Disadvantages: The two-incisionapproach is not popular because it istechnically difficult, has a steep learningcurve, and has a high intraoperativecomplication rate. In addition, there maybe injury to the abductor muscles49,51-53.

Anterolateral, Direct Lateral, orHardinge ApproachDirect lateral approaches include theHardinge approach, in which the gluteusmedius tendon is displaced with thevastus lateralis anteriorly and the hip isdislocated anteriorly54. Mallory et al.described a modified direct lateral ap-proach, in which the anterior portionof the gluteus medius is dissected anddisplaced anteriorly with the vastuslateralis55.

Limb length: Some surgeons per-form this approach with the patient inthe supine position, and this may havean advantage in terms of obtaining equallimb lengths. The approaches that dis-locate the hip anteriorly offer someadditional protection against dislocationcompared with posterolateral ap-proaches56. Therefore, slight laxity inthe hip to keep the lower limbs of equallength is acceptable.

Stability: The cumulative ten-yearrate of dislocation has been reportedto be 3.1% after anterolateral ap-proaches but 6.9% after posterolateralapproaches48,57,58.

Disadvantages: This approach vio-lates the abductor mechanism and issometimes associated with a postopera-

Fig. 3

Supine evaluation of limb length inequality is performedbymeasuring thedistance between the anterior superior iliac spine and the medial mal-leolus with a tape measure.

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tive limp. Damage to the superior glutealnerve can occur and leads to denervationof the muscles that it enervates59. Het-erotopic ossification is more commonthan it is with other approaches, and thisheterotopic bone has required removal in1% of patients58, a rate that is higher thanthat associated with other approaches.

Posterolateral ApproachThe posterolateral approach is the mostextensile of all approaches, allowingcomplete exposure of the femur andacetabulum. It is the most commonlyused approach in North America, pri-marily because it avoids damage to theabductor muscles50,60,61. Small-incisiontechniques have gained favor in recentyears61. The debate over the clinicalbenefit and the effect on limb length andstability of this approach is beyond thescope of this report61,62.

Limb length: When the postero-lateral approach is used, the limb lengthsare difficult to accurately measure withphysical examination or radiographs, sosome other means of determining limblength is necessary. Because of concerns

about postoperative dislocation, it isnot uncommon for the extremity to beoverlengthened during the hip arthro-plasty with this approach63.

Stability: The risk of dislocationassociated with the posterior approachis higher than that found with trans-trochanteric, anterolateral, and anterior-based approaches13,16,21,64. In a study ofover 21,000 primary total hip arthro-plasties, Berry et al. reported disloca-tion rates, at the time of a ten-yearfollow-up, of 3.1%, 3.4%, and 6.9% forthe anterolateral, transtrochanteric,and posterolateral approaches, respec-tively13. A meta-analysis by Masonisand Bourne suggested that the disloca-tion rate associated with the posteriorapproach is sixfold higher than thatobserved with a direct lateral approach64.Proper repair of the capsule and shortexternal rotators after a posterior ap-proach reduces the incidence of dislo-cation60,64-69. Furthermore, Kim et al.advocated preserving the external ro-tators during the posterior approach,a technique that resulted in zerodislocations70.

Disadvantages: The risk of injuryto the sciatic nerve with the posteriorapproach is reported to be 0.6%71,72.However, as a result of the proximityof the nerve with this approach, the riskof sciatic nerve injury is higher thanthat associated with all other surgicalapproaches59,70,73.

Surgical TechniqueImplant Positioning:Acetabular ComponentImplant malposition is a major con-tributor to instability and dislocation.Correct implant position decreaseswear and reduces the risk of disloca-tion, but other factors play a role in hipstability74,75. Multiple investigators haveattempted to define a safe zone ofacetabular component anteversion andinclination, or abduction. It is widelybelieved that the acetabular componentshould be placed in approximately 45!(40! to 60!) of abduction and should beanteverted 15! to 20! (Fig. 7)18. The safezone is 15! ± 10! of anteversion and40! ± 10! of abduction18. Total hiparthroplasty components that dislocate

Fig. 4-A Fig. 4-B

Fig. 4-A Anteroposterior radiographof a right femur in anexternally rotatedposition. A false femoral offset is seen.Fig. 4-BAnteroposterior radiographof thesame right femur with the hip in 20! of internal rotation. Note the marked femoral offset.

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anteriorly have mean anteversion andabduction angles that are greater thanthe safe zone, while those that dislocateposteriorly have mean anteversion andabduction angles that are less than thesafe zone76. The position of the acetab-ular cup is not the only factor affectinginstability and dislocation. Hassan et al.reported that 42% of total hip prosthe-ses in which the acetabular cup waspositioned outside the safe zone did notdislocate77. Rittmeister and Callitsisnoted that, while almost 20% of ace-tabular cups were positioned outside thesafe zone in their study, there was noincrease in dislocations in that group78.

Implant Positioning:Reference LandmarksLandmarks are useful in assisting withpositioning of the acetabular compo-nent. McCollum and Gray investigatedmultiple external reference points for

acetabular component positioning andfound that significant changes in pelvicposition and orientation occur whenthe patient is in the lateral decubitusposition79. Care must be taken to eval-uate the effects of body position whenusing external cues for orientation of theacetabular component during surgery.

Fixed anatomic landmarks, incontrast to external aiming devices, areindependent of patient positioning. Use-ful landmarks include the transverseacetabular ligament, the acetabular sulcuson the ischium, the most lateral promi-nence of the superior pubic rami (pubis),and the most superior aspect of theacetabulum80,81. These landmarks definea plane of orientation for acetabularcomponent positioning that providesstability within a safe arc of motion81. Anaverage cup position of 44! of abductionand 13! of anteversion can be achievedwith use of these landmarks81.

Computer navigation, orcomputer-assisted orthopaedic surgery(CAOS), has been proposed as a methodfor accurately determining correct ace-tabular component positioning. CAOSreduces outliers but is not totally reli-able82-84. The cost and technical aspects ofCAOS currently prohibit widespread use.

Implant Positioning:Femoral ComponentThe positioning of the femoral compo-nent affects limb length, offset, abductortension, and stability. All other thingsbeing equal, a distally placed femoralstem will result in a limb that is shorterthan that resulting from a more proxi-mally placed stem. The level of thefemoral component has an equallyimportant, albeit less obvious, effect onfemoral offset. Femoral offset is definedas the distance from the center ofrotation of the femoral head to a line

Fig. 5

Preoperative anteroposterior pelvic radiographmadewith the patient supine shows severe erosive arthritis of the right hip. Aline is drawn at the most inferior portions of the ischia, providing the pelvic reference line. A perpendicular line is drawnbilaterally from the transischial line to the superior aspect of the lesser trochanter to determine the limb length difference.The patient has a preoperative limb length discrepancy of 2.0 cm.

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bisecting the long axis of the femur.Reconstruction of the femoral offset isimportant for restoring the biome-chanics of the hip and specifically theabductor lever arm. Proper restorationof offset enhances hip motion andreduces the risk of dislocation85. A highfemoral neck resection can be combinedwith a short neck length to yield thesame limb length as provided by a lowfemoral neck resection combined witha long modular head. However, the firstcombination yields less femoral offsetand may be appropriate in the presenceof coxa valga. The second combinationyields greater offset and is better for hipswith coxa vara. Varus or valgus malpo-sitioning of the stem will increase ordecrease offset and should be avoided.Rotational alignment of the stem to theappropriate femoral anteversion influ-

ences the amount of hip motion that ispossible before impingement occurs aswell as abductor tension. Herrlin et al.noted that femoral anteversion wassignificantly reduced in hips that dis-located after total hip arthroplasty86. Theideal femoral anteversion is 15! to 20! inan osteoarthritic hip with otherwisenormal anatomy. Acetabular deformityor deficiency may dictate less thanideal orientation of the acetabularcomponent. To compensate for this, thefemoral component may need to beplaced in greater or less anteversion. Inrecognition of this possibility, the con-cept of combined acetabular and femo-ral component anteversion has beenintroduced. Using a mathematicalmodel, Widmer and Zurfluh deter-mined that the acetabular componentshould be in 40! to 45! of inclination

(abduction) and 20! to 28! of ante-version (forward flexion)87. This iscombined with femoral anteversionsuch that the femoral anteversion mul-tiplied by 0.7, plus the cup anteversion,should equal 37! in order to provide thegreatest range of motion without im-pingement. Modular femoral compo-nents of various designs that allowadjustments in offset and anteversionwithout limb lengthening are nowavailable from various manufacturers4,88.

There are some general rules ofthumb for placing a femoral stem in thecorrect position. The proximal-distalposition of the femoral stem is assessedin relation to the greater and lesser tro-chanters. Alternatively, the center of thefemoral head in relation to the tip of thegreater trochanter is noted. Additionally,the piriformis fossa can serve as a land-

Fig. 6

Preoperative templating canbeperformedwith useof computerized radiology software. Thenew, anatomic center of rotationis templated (A), and the acetabular implant size is determined with use of the hip that is not being operated on. Theappropriate femoral stem size and position are templated. The corresponding neck cut (B), prosthetic neck length (C), andlimb length difference are noted.

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mark for femoral neck resection. Whenthe posterior approach is used, thetemplated neck resection can be easilyreproduced by measuring the level ofresection from the top of the lessertrochanter. This landmark is easily visu-alized on preoperative radiographs andintraoperatively, even through limitedexposures. Woolson et al. described usingthe templated femoral neck and headsegment as a guide for placing the femoralstem89. By placing the femoral stem at thatosteotomy level, they achieved an appro-priate limb length in 97% of cases.

Soft-Tissue BalancingBy restoring femoral offset and limblength, proper balancing of the softtissues around the hip minimizes post-operative instability, pain, and limp90,91.Inadequate restoration of femoral off-set increases the risk of dislocation bydecreasing soft-tissue tension91. Exces-sive limb lengthening can result whenintraoperative instability due to inade-quate offset is inappropriately addressed

by increasing the neck length in anattempt to restore soft-tissue tension38.As mentioned, the combination of thesefactors is critical for understandingprosthetic hip stability19.

Better wear performance of theimplants has been observed after femo-ral head medialization and femoral shaftlateralization. In addition, restoration ofoffset is associated with better functionaland clinical results85,92,93. Bourne andRorabeck reviewed the availablemethods employed to restore offset90.The most common approach is the useof a lateralized (‘‘high-offset’’) femoralstem (Fig. 8). Another option is to usea lateralized acetabular liner. However,such liners decrease the abductor mo-ment arm, increase the joint reactiveforce, and result in accelerated poly-ethylene wear90. A lower-level neckresection and more distal femoral stemplacement combined with a longer necksegment can lateralize the femoral shaftwithout lengthening the limb. However,longer heads with skirts should be

avoided because they decrease motion asa result of impingement.

Concerns have been raised thatexcessive femoral lateralization may in-crease the incidence of thigh pain andtrochanteric bursitis or place unduestrains on the bone-cement or biologicinterfaces, leading to loosening. Thislatter concern has been refuted, and datashow that, when indicated, the use ofa lateralized stem improves the accuracyof hip soft-tissue reconstruction and doesnot increase thigh pain, trochantericpain, or loosening56. In fact, proper soft-tissue balancing, obtained with a lateral-ized stem, is associated with less thighand trochanteric pain94. However, over-lateralization should be avoided. Incavoet al. demonstrated that excessive later-alization led to a 15% incidence oftrochanteric pain95. The value of intra-operative tests of soft-tissue balance suchas the ‘‘shuck’’ or ‘‘drop-kick’’ test ishighly dependent on the surgical ap-proach, anesthetic technique, and sur-geon experience90. These tests, however,

Fig. 7

The so-called safe zone for orientation of the acetabular component. (Printed with permission of Joint Implant Surgeons, Inc.,New Albany, Ohio.)

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can provide the surgeon with an as-sessment of the overall tightness ofthe reconstructed hip. The shuck test isperformed by attempting to distract thetotal hip prosthesis in an inferior directionto assess the soft-tissue tension. The drop-kick test is performed by placing the hip inextension, flexing the knee to 90!, andreleasing the lower limb to assess theamount of recoil as the knee springs backtoward extension. In addition, intraoper-ative motion of the hip is important toevaluate for potential bone or prostheticimpingement and prosthetic stability.These intraoperative assessments coupledwith proper preoperative templatingshould allow the surgeon to restore properhip offset and limb length.

Measuring Limb LengthSubstantial limb length discrepancyoccurs after up to 3% of total hip

arthroplasties, but the clinical relevanceis not known69. The definition of clin-ically relevant limb length discrepancy isnot universally agreed on, with a rangebetween 6 and 35 mm having beenreported96-99. Most authors have agreedthat discrepancies of <1 cm are welltolerated97. Edwards et al. reportedaverage lengthening of 2.7 cm and4.4 cm in twenty-three total hip ar-throplasties complicated by peronealand sciatic nerve palsy, respectively71.White and Dougall reported thatlengthening of up to 35 mm does notaffect clinical results99. Edeen et al.reported that 32% of patients who hada total hip arthroplasty were aware ofa limb length inequality37. Relevant limblength discrepancy results in a limp,low-back pain, and functional impair-ment and is a major cause of litiga-tion38,100. There are several methods for

intraoperative assessment of limblength, with varied degrees of accuracy,technical difficulty, and expense. Manyof the methods involve an intraoperativemeasuring device, which may also enablemeasurement of offset41,63,101-103. Theseinstruments measure from a fixed pointon the pelvis to a fixed point on the femurand are used before femoral head dislo-cation and after total hip arthroplastyreconstruction. They are accurate if theposition of the limb before the dislocationis correctly reproduced for the post-arthroplasty measurement104. There isa learning curve with these devices as wellas the need for additional operative timeand expense. An average limb lengthen-ing of 3.4 mm was observed with the useof one specific device; limb lengtheningof >12 mm was observed in 5% of cases,and 7% had symptomatic lengtheningrequiring a heel lift103.

Fig. 8

Various methods of restoring offset with use of the femoral stem. (Printed with permission of Joint Implant Surgeons, Inc., New Albany, Ohio.)

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Alternatively, preoperative tem-plating and intraoperative ‘‘well-leg’’referencing for limb length is asaccurate as other methods, with fewradiographic outliers56. Preoperativetemplating is performed. The center ofthe acetabulum on the normal, con-tralateral side is identified with ace-tabular templates (Fig. 6). The femoralcomponent size and osteotomy levelare determined, and the neck length isselected. The level of the femoral neckosteotomy is referenced intraopera-tively with regard to the greater tro-chanter, the lesser trochanter, thepiriformis fossa, or the distance fromthe center of the resected head. Direct

measurement of limb length with thepatient supine is performed beforepositioning for the total hip arthro-plasty and preparation of the extremity.This measurement is correlated withthe preoperative templating. The pa-tient is positioned in the lateral decu-bitus position and the uninvolved lowerlimb is used as a reference, with therelative difference felt at the patellartendon (Fig. 9). The relative difference isreassessed after trial components havebeen placed. In one series of 410 patientstreated with a primary total hip arthro-plasty, an average lengthening of 3.9 mmwas seen and only two patients perceiveda limb length discrepancy56.

Implant-Related FactorsFemoral head size affects hip stabilityafter a total hip arthroplasty13,105-107.Dislocation rates for all approachesdecrease as femoral head size increasesfrom 22 mm to 32 mm13. Smith et al.reported no dislocations when a 38-mmhead had been used108. Cuckler et al. alsoreported no dislocations with use of38-mm heads, but 2.5% of total hipprostheses with a 28-mm head dislo-cated109. Peters et al. found no disloca-tions with 38-mm heads, a 0.4% ratewith 38 to 56-mm heads, and a 2.5%rate with 28-mm heads110. Smit studiedanatomically sized femoral heads (fem-oral heads with a size that was 6 mm less

Fig. 9

With the patient in a lateral position, the uninvolved lower limb is used to reference limb lengths intraoperatively. The pelvis needs to beperpendicular to the floor. With the feel symmetrically positioned, the patellar tendons are palpated, and the limb length difference isassessed. The goal is to have symmetric positioning of the patellar tendons with the pelvis and feet. (Printed with permission of JointImplant Surgeons, Inc., New Albany, Ohio.)

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than the acetabular size) in primarytotal hip arthroplasties, and reported nodislocations at the time of a one-yearfollow-up111. Others believe that a goodcapsular repair as well as a larger fem-oral head protects against a dislocation.Lachiewicz and Soileau found that,when a formal posterior capsular re-pair had been employed, there was nochange in the dislocation risk associatedwith 36 and 40-mmmetal femoral headscompared with that for historical con-trols with standard-sized heads112. De-spite the overall impressive reduction inthe dislocation rate associated with largefemoral heads in the above studies,Amstutz et al. reported a dislocation rateof 3.5% with use of large femoral headsin primary total hip arthroplasty113.However, Amstutz et al. demonstratedan advantage of using larger heads inrevision total hip arthroplasties113. Inaddition, the use of large heads increasesvolumetric wear, and a thinner poly-ethylene acetabular liner is needed toaccommodate the larger head. To avoidthe adverse mechanical and fatigueproperties associated with thin liners,implant companies commonly offeroffset liners to increase polyethylenethickness114. Offset liners may increasefemoral offset, and this affects the jointmechanics as previously discussed.

Management of InstabilityAn accurate and complete patient his-tory is critical for defining the cause ofhip instability. Operative records arereviewed to determine the surgicalapproach, type of soft-tissue repair, andspecific implant utilized, including themanufacturer’s implant stickers if pos-sible. The mechanism of the dislocationmay be evaluated according to the di-rection of dislocation and the positionof instability. Limb length, associatedskeletal conditions such as scoliosis andcontractures, neurological function ofboth the affected limb and the abduc-tors, and the overall neurological func-tion of the patient should be assessed.A thorough evaluation for infection isnecessary115.

Radiographic studies are essential.Anteroposterior and true lateral viewsof the hip and an anteroposterior view

of the pelvis are the minimal imagingstudies needed for these patients. Limblength differences, femoral offset, thestatus of the greater trochanter, and thecomponent orientation are noted. Apreoperative computed tomographyscan to evaluate the position of theacetabular cup can provide importantinformation regarding acetabular ver-sion116-119. Following evaluation anddefinition of the etiology of the dislo-cation, a treatment algorithm isestablished120,121.

The treatment options includeclosed reduction of the dislocated hipwith or without bracing, total hiparthroplasty component revision, ex-change of modular parts, cementinga liner into a well-fixed acetabular shell,bipolar or tripolar arthroplasty, utiliza-tion of a large femoral head, use ofa constrained liner, advancement of thegreater trochanter, and soft-tissue aug-mentation122-129. An understanding ofthe risk factors, causes of dislocation,and management options enables thesurgeon to effectively minimize theincidence of dislocation after totalhip arthroplasty as well as to establisha strategy for treating a patient withan unstable total hip prosthesis.

Treatment IndicationsSelection of the appropriate treatmentoption is guided by the cause and timingof the dislocation. Early dislocationsoccur within the first three to sixmonths after the operation, and in themajority of patients a single episode ofdislocation can be adequately treatedwith closed reduction130. The role ofcast-bracing or casting is controversial,and there are data supporting andrefuting the use of this treatment afterreduction of the hip123,126,129. Late dislo-cations are those that occur five years ormore after the index procedure. Patientswith a first-time late dislocation are athigh risk for recurrent instability131. Latedislocations have multiple possiblecauses, including polyethylene wear,trauma, decline in neurological func-tion, increased soft-tissue laxity, ormalposition of a total hip arthroplastycomponent132. Dislocations termed in-termediate occur between six months

and five years after the total hip arth-roplasty. Patients in whom this is thefirst dislocation can usually be managedwith closed reduction. Surgical man-agement should be considered forpatients with recurrent instability fol-lowing the initial closed reduction73,132.Successful operative management iscritically dependent on accurate identi-fication of the cause(s) of instability133.

Techniques and Results of RevisionTotal Hip Arthroplasty for InstabilityComponent revision is indicated whenimplants are seen to be malpositionedon radiographs, computed tomographyscans, or intraoperative evaluation118,119.Malpositioning of acetabular and fem-oral implants, limb length inequality,and improper femoral offset can becorrected and restored in a reasonablypredictable fashion with componentrevision11,58,120,134. Perhaps the easiest andmost attractive option for managingrecurrent instability in the presence ofimplants that appear to be in an ap-propriate position and alignment ismodular component exchange, or so-called dry revision135. This option is onlyindicated, however, if the componentsare reasonably well positioned136. In-creasing the head size and/or necklength and changing the acetabular linerare among the simplest solutions. Vary-ing degrees of success with this approachhave been reported in multiple smallseries. Toomey et al. successfully pre-vented recurrent dislocation with mod-ular component exchange in twelve ofthirteen hips, although three hips dis-located once during the follow-upperiod136. Importantly, these modularrevisions also included excision of softtissue and bone causing impingement inten hips136. Nine of the hips were con-verted to either a lipped-bearing implantor an implant with a higher degree oflipped bearing136. In another study, linerexchange was successful in 82% of casesof late instability associated with poly-ethylene wear137. In contrast, Barracket al. reported multiple complicationswith modular component exchange,including liner disassociation and im-pingement, instability, and femoral headdislodgement from the stem trunnion88.

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In cases of polyethylene-wear-relatedinstability, cementing a new liner intoa well-fixed shell may provide an al-ternative to complete revision if thecomponents are oriented correctly138,139.

High-walled liners can be valuablefor treating or preventing dislocation ofwell-positioned components, as reportedby Cobb et al.140. Similarly, in revisiontotal hip arthroplasty, an augmentationdevice can act as an elevated-rim liner.McConway et al. reported a 1.6% dislo-cation rate in 307 patients treated withrevision total hip arthroplasty with aposterior lip-augmentation device141.Adding an augmentation device to theexisting liner or socket has also beendescribed and is effective in certaincases142-144. Currently, high-wall or lippedliners are used more sparingly because ofconcerns regarding impingement, wear,and limited hip motion.

Large femoral heads increase thehead-neck ratio, thereby increasing therange of motion before impingementoccurs, and increase the jump distancerequired for the head to dislocate107. Ina series in which large femoral heads(36 mm and larger) were used, Beauleet al. reported that >90% of the hipshad no more instability after an averageduration of follow-up of 6.5 years and onlyone had recurrent instability145. Amstutzet al. reported that the dislocation rateafter revisions for recurrent instability washigher than that after revisions for otheretiologies113. More troubling results werereported by Skeels et al., who observeda 17% rate of recurrent dislocation inpatients who had undergone revisionsurgery with use of a femoral head thatwas 36 mm or larger146.

Another, less commonly utilizedstrategy to manage instability involvessoft-tissue augmentation, or reinforce-ment of the hip abductor muscles and/orthe posterior aspect of the hip capsule19.Reconstructions with an Achilles tendonallograft and a bone block, fascia lata,or a synthetic ligament have all beenreported147-149. Indications for these pro-cedures are unclear but may includedeficiency of the hip abductor muscles orposterior aspect of the hip capsule in thesetting of well-positioned, well-fixedtotal hip arthroplasty components.

Trochanteric advancement hasbeen advocated for patients with well-positioned, well-fixed total hip arthro-plasty implants124,150,151. Nonunion of thegreater trochanter is a major concernand trochanter-related hip pain is com-mon. Ekelund124 and Kaplan et al.151 in-dependently reported 80% success rateswith use of this approach in twenty-onepatients each with recurrent dislocationand properly oriented components.Similarly, trochanteric osteotomy andadvancement can be utilized for complexprimary total hip arthroplasty to enhancestability152.

Bipolar arthroplasty is based onthe principle of increasing the overallrange of motion with articulation at twodifferent bearing surfaces133,153,154. Thisprovides a greater safe arc of motionbefore dislocation occurs and opti-mizes head-neck ratios while providinga larger jump distance. Parvizi andMorrey reported the elimination ofrecurrent dislocation in 81% of twenty-seven hips133. Attarian153 and Ries andWiedel154 achieved 100% success usingthis technique. Medial and/or superiormigration of the prosthesis, with re-sultant groin pain, is a concern if thistechnique is used.

Unconstrained tripolar hip ar-throplasty utilizes a bipolar head toarticulate with an acetabular shell andliner, and this combination increasesthe head-neck ratio and the jumpdistance155-157. Grigoris et al.156 and Beauleet al.155 used an unconstrained tripolarimplant to successfully treat instabilitywithout compromising acetabular fixa-tion in 95% of their cases. Levine et al.reported a 93% success rate in a series ofthirty-one patients in whom an unstabletotal hip prosthesis had been treated withan unconstrained tripolar construct157.

The final salvage option involvesthe use of a constrained acetabularliner19,158-170. Indications for this tech-nique include hip abductor deficiency,neurological impairment, low-demandpatients with well-fixed components,instability for which the cause cannotbe determined, and persistent intra-operative instability19,122,127,158,167,168. Con-strained acetabular liners reduce thehip motion prior to impingement and

therefore increase the risk of impinge-ment and the acetabular shear stresses,which could lead to accelerated wear,loosening, or failure of fixation. Theseimplants can be cemented into a well-fixed acetabular shell to reduce themorbidity of revision total hip arthro-plasty138. Callaghan et al. reported nodislocations and two liner failures (a94% success rate) with the use of thistechnique in patients with a well-fixed,well-positioned cementless acetabularshell163. This procedure is considereda low-morbidity treatment option in thesetting of a well-fixed, properly orientedacetabular component, especially inolder, low-demand patients163.

Favorable results with the use ofconstrained devices have been reportedin several studies, but the use of thesecomponents should be consideredonly if no other treatment options areavailable171. At an average of 10.2 yearsafter the use of fifty-six constrainedtripolar devices, Goetz et al. reporteda 7% failure rate secondary to recurrentdislocation, osteolysis, or aseptic loos-ening164,165. Bremner et al. reportedsimilar results, with a 6% failure ratesecondary to recurrent dislocation orliner failure at 10.2 years161. There isconcern about the stability of fixationof constrained devices. Shrader et al.noted that, while no dislocations wereseen, there were acetabular cup radio-lucencies in 14% of their cases169. Suand Pellicci reported a 98% rate ofsuccess in terms of preventing insta-bility in eighty-five hips with a con-strained tripolar implant170. There aremodes of failure specific to tripolarconstrained devices172-174. Guyen et al.reported forty-three failures of tripolarconstrained devices, with four types offailure, including the bone-implantinterface, the mechanism holding theconstrained acetabular liner to themetal shell, the locking mechanism ofthe bipolar component, and dislocationof the head at the inner bearing173.Methods for closed reduction of a con-strained component have been de-scribed, but long-term outcomes havenot yet been reported175.

One of us (K.R.B.) and colleaguesreported on 755 alternatively designed

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constrained total hip arthroplastycomponents with a capture mechanismand locking ring design159. The dislo-cation rate for the 667 hips followedfor ten years was 17.5%, and asepticloosening of the cup and stem were alsomajor long-term causes of failure thatrequired a reoperation159. Newer de-signs allowing greater hip motion priorto impingement have been introduced.One of us (K.R.B.) and colleaguesreported a 99% rate of success in termsof preventing recurrent dislocation ina group of eighty-one total hip arthro-plasty revisions done with a novelconstrained device160.

OverviewIn conclusion, the intraoperative chal-lenge of achieving stability and limblength equality after total hip arthro-plasty starts with preoperative plan-ning, including physical examination,radiographic evaluation, templating,and aligning patient and surgeon ex-pectations. Each surgical approach hasadvantages and disadvantages in termsof stability and limb length. It is theresponsibility of the surgeon to befamiliar with the drawbacks and bene-

fits of each approach and to utilizea method that most easily accomplishesthe goals of a stable prosthetic con-struct, hip stability, and restoration oflimb length equality. Familiarity andexperience with a total hip arthroplastytechnique reduce the risk of dislocationand limb length inequality. Intraoper-atively, the prosthetic design includingthe femoral head size and femoraloffset, component orientation, andreconstruction of the hip soft tissuesare the critical variables for achievingsuccess. Preoperative radiographictemplating is paramount, and intra-operative maneuvers to determinelimb length are important for obtainingthe best result. Dislocation continuesto be a major mode of failure of totalhip arthroplasty. Obtaining a stable hipat the time of the initial total hiparthroplasty reduces the risk of thiscomplication.

Keith R. Berend, MDMichael J. Morris, MDJoint Implant Surgeons, Inc.,7277 Smith’s Mill Road, Suite 200,

New Albany, OH 43054.E-mail address for K.R. Berend:[email protected]

Scott M. Sporer, MDDepartment of Orthopedics,Rush University Medical Center,1725 West Harrison Street,Suite 1063,Chicago, IL 60612

Rafael J. Sierra, MDDepartment of Orthopedic Surgery,Mayo Clinic College of Medicine,200 First Street S.W., Rochester,MN 55905

Andrew H. Glassman, MD, MSDepartment of Orthopedics,Ohio State University,1491 East Broad Street, Columbus,OH 43205

Printed with permission of the AmericanAcademy of Orthopaedic Surgeons. This article,as well as other lectures presented at theAcademy’s Annual Meeting, will be available inFebruary 2011 in Instructional Course Lectures,Volume 60. The complete volume can beordered online at www.aaos.org, or by calling800-626-6726 (8 A.M.-5 P.M., Central time).

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