PREOPERATIVE PLANNING OF REVISION TOTAL KNEE

ARTHROPLASTY J.J. Prosser, D.O.

Introduction

• Of the 200,000 TKAs performed in the U.S. in 1996, 50,000(1/4) were performed in patients under 65y/o

• 20,000 revision TKAs in 1996 alone

• More and more individuals at risk for implant failure

Implant Failure

• 2 causes of implant failure;– 1.aseptic loosening– 2.wear related osteolysis

Implant Failure continued…

• Both result in loss of periprosthetic bone

• Stability of revision implant depends on quantity and quality of metaphyseal bone

• ID bone defects on prerevision radiographs is a crucial part of preoperative planning

Goals

• Shorten the learning curve of the implant system– Appropriate size- fracture vs. loosening

• Correction of leg length discrepancies – Correct during surgery

• Formulate alternative plans if procedure of choice can not be performed - special equipment on standby

Results

• Proficient preoperative planning leads to a reduction in surgical time

• It minimizes or eliminates the need for repeated steps

Planning: X-Rays

• Radiographs of good quality

• Known magnification

Radiographs of Good Quality

• AP and lateral

• AP– Must include sufficient amount of proximal

femur for templating– Must provide a scale for magnification– Proper rotation- internal rotation of the hip until

the head and neck are parallel with the cassette

Rotational Error

• Degree of error in sizing is a function of the rotational distortion

• Closer to an ideal AP projection less error in templating

Leg Length Discrepancy

• Draw a line at the level of the ischial tuberosities

• Relationship between this line and the lesser trochanter allows for assessment

• Measure and record any discrepancies

Template of the Acetabulum

• Position template so that the center of the prosthesis reproduces the center of rotation of the head

• Typically the inferior margin of the acetabulum lies at the level of the obturator foramen

Sizing of the Acetabulum

• Implant should contact as much of the acetabular bone as possible

• Do not remove excessive amount of subchondral bone

• Medial position should be at the tear drop

• Eccentricities can be managed with bone grafting or eccentric cement mantles

Position of the Acetabular Component

• 15 degrees of anteversion:– measured as the largest distance of the cup

opening on the AP centered on the cup

• 40 degrees of inclination:– measured as the angle between the longitudinal

axis of the cup opening and the tear drop line

Template of the Femur• Must allow for cement

mantle with cemented arthroplasties

• Size and location of mantle are discretion of the surgeon

• Press fit must fill the proximal canal

• Size determined by template that best fits the upper femur

Head and Neck Size

• No leg length discrepancy:– superimpose the center of the prosthetic head

and femoral head to the acetabular center of the hip

• Leg length discrepancy:– distance from the acetabular center to the

femoral center should equal the discrepancy

Type and Size of the Femoral Component

• Determined by adjusting medial side of the component to medial wall of the canal

• The trochanteric line of the template is placed at the apex of the greater trochanter

• Size determines the amount of offset

• Larger the prosthesis the larger the offset

Neck Resection• Template will indicate the neck resection level

– easily measured from the proximal aspect of the lesser trochanter

– reproduced in surgery with adequate exposure– can also utilize the relationship between the tip of the

greater trochanter and the prosthetic femoral head

• Any measurements must be done with a ruler whose scale is magnified the same amount as the x-ray

Completed Template

• Line B to C: leg length discrepancy

• Point D: center of the acetabulum

• Point E: center of femoral head

• Line F: intended level of resection

Final Planning

• Most templates include a magnified scale• Lateral x-ray templating done in a similar fashion• Size of the medullary plug can be determined by

measuring the canal at the level of the stem• Templating the non-operative side in complicated

revision cases is often helpful to assess the need for special equipment or bone graft

Checking the Plan• Fitting of trial components and instrumentation• In uncemented THAs criteria for the tightness of

the stem– apply predetermined rotation and extraction forces

• Postoperative x-rays: – compare components, cuts, and leg length to preop

plan

– check acetabular placement and orientation as well as fit of the femoral stem

Summary

• Preoperative planning is a necessary first step

• Shorten learning curve of prosthetic system

• Minimize intraoperative guess work

• Decrease surgical time

• Allows for precision and reproduceability

Eggli S, Pisan M, Muller ME

• The value of preoperative planning for total hip arthroplasty.

• JBJS. 1998 May;80(3):382-390.

History

• Both Charnley and Muller emphasized the importance of preop radiographs in deciding the type and size of prosthesis, in achieving the correct position and orientation of the components, in equalizing leg length and in reducing intraoperative complications

Study

• 100 consecutive patients

• THR secondary to idiopathic OA from 1985-1988

• 45 men and 55 women

• Mean age of 66

Three basic steps

• Appropriate type and size of prosthesis– 3 neck lengths

Anatomic position and orientation of the acetabular component

40 degrees of inclination

15 degrees of anteversion

Restoration of leg lengths

Results

Stem- 98% type

92% size

Acetabulum- 100% type

90% size

Leg length- 0.9 preop

0.3 postop

Discussion

• Used cemented femoral component in 90% of cases

• More pre and post op agreement

Knight JL, Atwater RD.

• Preoperative planning for total hip arthroplasty. Quantitating its utility and precision.

• J Arthroplasty. 1992;7 Suppl:403-409.

Study

• 110 primary THA

• Cemented and uncemented

Results

• Preop estimate of magnification differed from actual magnification

• Preop bone morphology did not correlate with type of femoral stem fixation

• Preop estimate– Acetabular cups- 62%– Cemented stem- 78%– Cementless- 42%Leg length equalization

70%

Discussion

• Surgeons need better methods to estimate bone morphology and magnification from preop radiographs

• Preop planning may not reduce intraoperative complications