Issue 20

What’s NewZan educational service from Zimmer

The outcomes achieved with

standard femoral implants used in

total knee arthroplasty (TKA) are

satisfactory for most patients,

however there are significant reasons

for developing a gender specific knee

implant for women.

Standard implants can be a

contributing cause of post-operative

morbidity in women, said Dr Robert

E. Booth Jr, Clinical Professor of

Orthopaedic Surgery at the

University of Pennsylvania School

of Medicine, Philadelphia.

“ Even though most women are

happy with their results after joint

replacement, some still have

residual pain in the front of the

knee…or say their artificial knee just

doesn’t feel or move the way their

own knee did when it was healthy,”

said Dr Robert Booth.

The problem is that while standard

implants are currently available in

variable sizes, they are based on the

combined average of male and

female anatomy that doesn’t take

into account shape-related

anatomical differences between

the genders, he said.

Standard implants are currently available in variable sizes, but are based on the combined average of women and men’s anatomy that don’t take into account shape-related gender differences

These differences have been

reported in a number of studies1,2,3,4,5

but have been difficult to quantify

with a high degree of precision and

accuracy. As a result women

undergoing TKA have continued

to be treated simply as smaller

versions of men with regards to

implant selection, said Dr Booth.

The growing number of women

who require TKA has also led to

an increased demand for improved

outcomes. Women made up 58.1%

of primary TKA in 2004-5, the fastest

growing major joint replacement

in Australia (18.1% increase from

2002-3 to 2004-5), according to

Australian Joint Replacement

Registry data.6 This trend is set to

increase with the ageing population.

Research also shows that women are

three times less likely than men to

undergo arthroplasty although they

suffer from a higher prevalence of

arthritis, more severe symptoms and

greater disability.7 With an implant

“shaped to fit women’s anatomy”

there is an expectation that “far

more women will want to consider

knee replacement,” said Dr. Booth.

In addition, a better fitting implant

may require fewer intraoperative

adjustments to accommodate

anatomical differences, contributing

to reduced operating time.

In this issueThe need for a knee implant

designed specifically for women ..... 1

Best of both worlds - The Zimmer

Gender Solutions High-Flex Knee .... 2

Three distinct differences

in shape ......................................... 2

(1) Modified Aspect Ratio – to better

fit the shape of a woman’s

trapezoidal distal femur ............. 2 - 3

(2) Modified Anterior Flange –

reduced thickness to equal the

amount of bone removed and avoid

overstuffing .................................... 4

(3) Lateralised Sulcus – to provide

more natural patella tracking for a

woman’s greater Q-angle ................ 4

The need for a knee implant designed specifically for women

Dr Robert E. Booth Jr. MD Clinical Professor of Orthopaedic Surgery at the University of Pennsylvania, School of Medicine, Philadelphia

� What’s NewZ Issue �0

“ Knee implants have been

functioning very well for men

and women, but we want to meet

women’s unique needs by making

knee replacements that feel, fit

and function even better,”

said Dr Booth.

“ The Gender Solutions Knee

is the best of both worlds. It’s

based on the NexGen® Knee,

a highly successful implant with

great mechanics and ten years

of clinical success. Only the shape

is different, to make it feel more

natural,” he said.

The Gender Solutions Knee is based on the NexGen® Knee, a highly successful implant with great mechanics, only the shape is different, to make it feel more natural

Three distinct differences in shapeThe Gender Solutions High-Flex

Knee has the potential to achieve

substantial intraoperative and

post-operative benefits for TKA

patients by addressing three

distinct shape-related differences

of a woman’s knee.

(1) Decreased medial-lateral

(M/L) dimension reduces

implant overhang

Previous research has reported

differences in knee anatomy between

women and men.1,2,3,4,5

Poilvache et al conducted

measurements including trans-

epicondylar width and AP dimensions

before and after resection in their

1996 study of 54 female and 46 male

knees undergoing TKA.1

They found “a significant, although

small, difference…between males

and females in the ratio between the

transepicondylar width and the mean

height of the condyles…suggesting

that females had narrower femurs

than males. The difference was even

greater when subtracting the anterior

extent of the trochlea (anterior cuts)

from the height of the condyles.”1

In 2002 Chin et al conducted intra-

operative measurements of 100

female and 100 male femurs after

making the distal femoral cut. Their

data found that “for any given A/P

femoral dimension, women tend to

have a narrower M/L dimension than

men, independent of A/P height.”2

In 2003 Hitt et al also measured

the amount of M/L implant overhang

in both genders in addition to

confirming that the female femurs

were narrower. They observed that

the overhang was greater in females

regardless of the brand of implant.3

Despite the insights achieved by

these and other studies, the methods

used for the intra-operative

measurements, including calipers

and rulers, were limited by a lack

of precision and variability.

Zimmer® Gender Solutions™ NexGen® High-Flex Knee

Best of both worlds: The Zimmer® Gender Solutions™ NexGen® High-Flex Knee

�What’s NewZ Issue �0

Research conducted for Zimmer

by a team led by Mohamed Mahfouz,

Ph.D., director of Computational

Bioengineering at the University

of Tennessee, Knoxville, set

out to accurately quantify

these differences.

A continually growing knee bone

atlas based on more than 800 knees

and patellae was created for Zimmer

by Dr Mahfouz. Bone atlases have

been used in the past to create knee

replacements; however the data

were two-dimensional and

interpreted differently.

In contrast, Dr Mahfouz and co-

researchers utilised three-

dimensional computed tomography

(CT). Data was collected for the

femoral condyle dimensions to

create highly detailed virtual

blueprints of the knee as it looks

after resection when the bone is

ready for the implant.

Based on findings from this research

that was reported earlier this year,

a graph that plots the mid-box

medial-lateral (M/L) dimension

versus the overall anterior-posterior

(A/P) dimension confirms the

findings of the earlier research that

the average female knee had a

decreased M/L dimension compared

to men’s or ‘the male knee’ with the

same overall anterior-posterior

(A/P) dimension.8

The female knee has a decreased M/L dimension when compared to the male knee anatomy.

A consequence of the narrower M/L

dimension is that female knees were

found to be more trapezoid-shaped,

whereas male knees were

characterized by a rectangle-shape.

Overhang is reduced at the M/L interface leading to potential intraoperative and post-operative benefits.

The decreased M/L dimension was

incorporated into the Zimmer®

Gender Solutions™ High-Flex Knee

design, with the aim of reducing

implant overhang, which has the

potential to achieve intraoperative

and post-operative benefits.

Overhang can press on or damage

surrounding soft tissues and may

possibly cause pain and reduced

function. Intraoperative adjustments

to compensate for the overhang are

often required.2

Overhang can occur because

a patient’s implant size is usually

selected based on the A/P

dimension, which is the key

to achieving normal knee kinematics

and maximizing the potential for

full flexion.

However standard non-gender

specific implant designs that provide

a good A/P fit are often too wide in

the M/L dimension leading to

overhang at the distal, anterior

and posterior M/L interfaces.1

Femoral condyle mid-box medial-lateral (M/L) dimension versus overall anterior-posterior (A/P) dimension

Difference in the distal femur shape: Females (pink) compared to males (blue)

A/P

M/L Mid-Box

A/P

A standard implant may overhang at the distal, anterior and posterior M/L interfaces compared to the reduced or eliminated overhang of the Gender Solutions High-Flex Knee

The new computational method devised by Zimmer for predetermining the contour of a resected bone and fit of an implant that was used in the development of Gender Solutions High-Flex Knee

� What’s NewZ Issue �0

(�) Reduced anterior flange

thickness decreases

implant protrusion

Research has shown that the female

knee has a less pronounced anterior

condyle compared to males due to

decreased bone thickness.2, 9

In the 1996 Poilvache et al study

cited earlier, measurements were

also taken of the resected thickness

of the anterior condyles. The average

lateral and medial condyle thickness

in males was statistically significantly

greater than for females.1

Zimmer data has confirmed the

gender difference in thickness –

0.8 mm less in females on the lateral

condyle and 1.3 mm less on the

medial condyle.9 In addition the M/L

dimension of the resected anterior

flange bone is narrower in women.9

The Gender Solutions High-Flex Knee

addresses the distinctive anterior

condyle differences by reducing the

anterior flange thickness as well

as narrowing the implant anteriorly

decreasing the M/L dimension

of the implant.

To compensate for reduced anterior

flange thickness, the anterior

condyles have been lowered and

the patella sulcus has been

recessed, while retaining the

NexGen® patella articulation.

Reduced anterior flange thickness of the implant decrease anterior protrusion of the joint

The advantage of the reduced

anterior flange thickness of the

Zimmer® Gender Solutions™

High-Flex Knee implant is decreased

anterior protrusion of the joint,

which can reduce pain from

overstuffing the joint. In addition

to reducing bulkiness, patients may

experience improved function and

there may be less need for

intraoperative adjustments.

This is due, in part, to a reduction

in the stress on the patella ligament

as the knee moves into overstuffing

on angles. To compensate for this,

a surgeon’s only option (in the past)

was to translate the entire femoral

component inferiorly. This

intra-operative compromise

could result in either matching

of the femoral anterior cortex or

overstuffing the flexion gap. A knee

designed with a thinner anterior

flange demonstrates the need

for this compromise.

(�) Greater Q-angle reduces implant

patella maltracking

While patella maltracking remains

a concern for all patients following

TKA, it is a particular concern for

females due to their patellofemoral

joint biomechanics, which

predisposes them to pain associated

with patella malalignment and

quadriceps weakness.10

One of the main features of female

patellofemoral joint biomechanics

is a statistically significant higher

quadriceps femoral angle (Q-angle)

compared to men.11,12,13, 14

Aglietti et al found a 3º greater

Q-angle in the women compared

to males when measured with a

goniometer in supine in their 1983

study of 240 male and female

university-aged subjects.11

Gender Solutions High-Flex

Femoral Implant

Traditional implant

The Gender Solutions High-Flex Knee addresses the distinctive anterior flange differences

Traditional implant Gender Solutions High-Flex

Femoral Implant

�What’s NewZ Issue �0

In 1990 Hsu et al measured the

Q-angle of 60 knees of subjects aged

25-40 years and 60 aged 41-60 years

using x-rays in standing, full weight-

bearing position. They found a 3.7º

difference in the women in the young

group and 2.7º in the older group.12

Two years later Woodland et al

measured the Q-angle of 269 male

and 257 female university-aged

subjects in supine and standing

using a goniometer and found it was

3.1º greater for the females in supine

and 3.4º in standing.13

Emulating the greater Q-angle reduces patella maltracking

The Zimmer® Gender Solutions™

High-Flex Knee emulates the Q-angle

difference by increasing the patella

sulcus angle 3º laterally. The

potential benefit is reduced

patella maltracking and decreased

associated post-operative morbidity.

The early Australian experienceThe first orthopaedic surgeon in

Australia to use a Zimmer® Gender

Solutions™ High-Flex Knee implant,

Dr Brett Courtenay, from the St

Vincent’s Clinic in Sydney, says the

improved fit was immediately clear

to see in the operating theatre.

“ In many patients there is

considerable overhang of the

femoral component when the

correct sizes [of standard implants]

are used. This implant clearly

reduces this overhang without any

changes to the bone cuts. The

decision is made at the time of

using the trial implant,” he says.

“ Having used the new implant I

would feel fairly confident that it

will allow me to use a more correct

sized prosthesis with my patients.”

Dr Courtenay says “in the past

when I was worried about overhang

I tended to slightly undersize which

is generally not a good thing to do

because it can lead to instability in

flexion and less efficient movement

of the prosthesis.”

In the past when I was worried about overhang I tended to slightly undersize.

“ The trouble is that if you put a

smaller prosthesis in it might feel

stable when the knee is extended

but it is not going to be as stable

when the knee is flexed.”

One of the key benefits offered by

the Gender Solutions™ High-Flex

Knee implant is increased variability.

“ Previously we had one prosthesis

that was meant to fit all. Now

we’re going to have slimmer and

broader prostheses. I feel very

comfortable that this step is one

to address a problem that we have

had with sizes of prostheses,”

says Dr Courtenay.

“ It allows me to use the correct

size according to the measurements

without the risk of an overhang,”

he says.

The Gender Solutions High-Flex Knee allows me to use the correct size according to the measurements without the risk of an overhang.

Greater Q-angle of the Gender Solutions High-Flex Knee

� What’s NewZ Issue �0

“ Time will tell how effective it will

be with regards to functional

outcome. But we’re coming from

a prosthesis that really has

demonstrated that it is very good

at returning bend to the knee.

The [Zimmer High-Flex Knee]

prosthesis, the basic one in its

design, is safe to 155 degrees. The

Gender Solutions is another step

in that process and very definitely

warrants encouragement.”

Additional sizes and half sizes don’t address the gender issueEven though many standard implants

are provided in additional sizes or

half sizes, such as the Stryker

Triathlon implant, they do not

address the main gender issue,

which is the shape difference

between women and men.

Patient selectionThe Zimmer® Gender Solutions™

NexGen® High-Flex Knee may

be suitable for:

Female patients requiring

a primary total knee

arthroplasty (TKA)

Some female patients

requiring revision total

knee arthroplasty (TKA),

to the extent that primary

TKA implants are currently

used by the surgeon for

revision TKA

A small proportion of male

patients with a narrower M/L

dimension for a given A/P

when compared to the rest

of the male population.

•

•

Zimmer systems and techniquesThe Zimmer® Gender Solutions™

NexGen® High-Flex Knee not only

retains all of the elements of

Zimmer’s clinically successful

NexGen® standard knee

system, but also provides the

added bonus of safely

accommodating up to

155 º of flexion for patients with

the desire and ability to achieve

overstuffing on.

In addition it is compatible with

other Zimmer surgical

techniques, including Minimally

Invasive Solutions™ procedures,

which typically offer smaller

scars, shorter hospitalization

and quicker rehabilitation

and recovery.

Femoral condyle mid-box medial-lateral (M/L) dimension versus the overall anterior-posterior (A/P) dimension of standard implants and Gender Solutions High-Flex Knee.

As seen on a graph that plots the

femoral condyle mid-box medial-

lateral (M/L) dimension versus

overall anterior-posterior (A/P)

dimension, standard implants still

tend to follow the same curve line,

which is based on the average of

both women and men, whereas the

Gender Solutions High-Flex Knee

stands out as having a decreased

M/L dimension.

In addition, standard implants do not

have the decreased anterior flange

thickness and greater Q-angle that

the Gender Solutions High-Flex Knee

implant exhibits.

�What’s NewZ Issue �0

NOTES AND QUOTES“ We have known for years that there are differences between men and women.

And we’ve accommodated…that with our surgery technique. [But] as our

patients are becoming more active, wanting a better result, expecting more

for their total knee replacements I think we have to accommodate that by

having gender specific knees.”

Dr Richard A. Berger, Rush University Medical Center, Chicago

“ If we think back twenty-five years ago, we had knees that weren’t even left

and right. Twenty-five years ago, we figured you had to have left and rights.

Twenty-five years later, we find out you have to have male and female…we’re

realizing that the size difference and the ratio of medial to lateral and anterior

to posterior [femoral condyle dimensions] are different for females.”

Dr Aaron A. Hoffman, University of Utah Medical Center, Salt Lake City

“ I think if a surgeon questions whether or not if this is a real advance…he can

answer that question in the operating room as he puts a trial prosthesis on

and sees that it overhangs significantly and then puts the gender specific

prosthesis on and sees that it conforms accurately to the dimensions of that

particular knee. He’ll be instantly convinced that there is a place for this

particular prosthesis.”

Dr Kim C. Bertin, Utah Hip and Knee Center, Salt Lake City

The Zimmer® Gender Solutions™ High-Flex Knee Advantage

A gender specific femoral implant design helps to meet the expectations

of female patients by providing a better anatomical fit leading to

potential benefits:

Reduced pain

Decreased feelings of bulkiness and overstuffing

Improved function and biomechanics, particularly patella tracking

Reduced need for intraoperative adjustments

•

•

•

•

REFERENCES

1. Poilvache PL, Insall JN, Scuderi GR, Font-Rodriguez DE. Rotational landmarks and sizing of the distal femur in total knee arthroplasty. Clin Orthop Relat Res 1996 Oct;331:35-46.

2. Chin KR, Dalury DF, Zurakowski D, Scott RD. Intraoperative measurements of male and female distal femurs during primary total knee arthroplasty. J Knee Surg 2002 Fall;15(4):213-7.

3. Hitt K, Shurman JR 2nd, Greene K, et al. Anthropometric measurements of the human knee: correlation to the sizing of current knee arthroplasty systems. J Bone Joint Surg Am, 2003;85-A Suppl 4:115-22.

4. Vaidya SV, Ranawat CS, Aroojis A, Laud NS. Anthropometric measurements to design total knee prostheses for the Indian population. J Arthroplasty 2000 Jan;15(1):79-85.

5. Urabe K, Miura H, Kuwano T, et al. Comparison between the shape of resected femoral sections and femoral prostheses used in total knee arthroplasty in Japanese patients: simulation using three-dimensional computed tomography. J Knee Surg 2003 Jan;16(1):27-33.

6. Graves S, Davidson D, Ingerson L, et al. Australian Orthopaedic Association National Joint Replacement Registry annual report. Adelaide: Australian Orthopaedic Association, 2006.

7. Hawker GA, Wright JG, Coyte PC, et al. Differences between men and women in the rate of use of hip and knee arthroplasty. N Engl J Med 2000 Apr 6;342(14):1016-22.

8. Mahfouz M, Booth R Jr, Argenson J, et al. Analysis of variation of adult femora using sex specific statistical atlases. Presented at: Computer Methods in Biomechanics and Biomedical Engineering Conference; 2006

9. Data on file at Zimmer.

10. Csintalan RP, Schulz MM, Woo J, et al. Gender differences in patellofemoral joint biomechanics. Clin Orthop Relat Res 2002 Sep;(402):260-9.

11. Aglietti P, Insall JN, Cerulli G. Patella pain and incongruence. I: Measurements of incongruence. Clin Orthop Relat Res 1983 Jun;(176):217-24.

12. Hsu RW, Himeno S, Coventry MB, Chao EY. Normal axial alignment of the lower extremity and load-bearing distribution at the knee. Clin Orthop Relat Res 1990 Jun;(255):215-27.

13. Woodland LH, Francis RS. Parameters and comparisons of the quadriceps angle of college-aged men and women in the supine and standing positions. Am J Sports Med 1992 Mar-Apr;20(2):208-11

14. Livingston LA. The quadriceps angle: a review of the literature. J Orthop Sports Phys Ther. 1998 Aug;28(2):105-9.

� What’s NewZ Issue �0

The Zimmer® Gender Solutions™ High-Flex KneeTotal Knee Arthroplasty implant that emulates three distinct and scientifically

documented anatomical shape features of women’s knees

(1) Decreased mid-box medial-lateral (M/L) dimension versus overall A/P

dimension leads to an improved fit of the implant leading to reduced

overhang at the distal, anterior and posterior M/L interfaces →

(�) Reduced anterior flange thickness of the implant decreases

anterior protrusion →

(�) Greater Q-angle of the implant improves patella tracking →

A Special Thank YouOn behalf of John Cooper, Vice President, Australia, New Zealand and India we

would like to thank our guests for celebrating the launch of the revolutionary

Gender Knee at the Zimmer ‘Art of the Knee’ Gala Dinner held 10th October at

the National Gallery of Australia.

For those people who were unable to attend the event, Professor Betty

Churcher introduced guests to the mysterious yet fascinating world of academic

art history. Guests experienced an informative and entertaining journey that

encompassed the representation of the human anatomy from the 10th century

Nordic tradition to present pieces produced by Australian contemporary

photographer Bill Henson. How artists depicted female nudes was not only

interesting but it was apparent that art and science can portray entirely

different facts about the human body.

To discuss the scientific attributes was international guest speaker Dr Robert

Booth, Chief of Orthopaedic Surgery, Pennsylvania Hospital, Philadelphia, USA.

Dr Booth was one of the key orthopaedic surgeons on the Zimmer® Gender

Solutions™ NexGen® Knee Development Team. His presentation titled

‘The EVEolution’ of the TKA’ presented our learned audience with hard

clinical evidence that supported the design, development and manufacturing

of a femoral prosthesis shaped to fit the female anatomy.

The positive feedback that we have received from both orthopaedic surgeons

and partners alike has been overwhelming. Once again, we would like to thank

you for being a part of our very special evening.

Gender Solutions High-Flex

Femoral Implanton female bone

Traditional implant overhangs female bone

Gender Solutions High-Flex

Femoral ImplantTraditional implant

(1)

(�)

(�)

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