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ORIGINAL ARTICLE

The Impact of Component Design on Lateral Patellar Release Rates in Female Primary Total Knee Arthroplasty

William J. Long*,  Giles R. Scuderi** , Jennifer L. Cook #

*Attending Orthopaedic Surgeon,Insall Scott Kelly Institute for Orthopaedics and Sports Medicine, 210 East 64th Street, New York,
**Director, Attending Orthopaedic Surgeon
, Insall Scott Kelly Institute, 210 East 64th Street, New York
#
Florida Joint Replacement and Sports Medicine Center ,5243 Hanff Lane ,New Port Richey ,FL 34652

Address for Correspondence:  

William J. Long MD FRCSC

Attending Orthopaedic Surgeon

Insall Scott Kelly Institute for Orthopaedics and Sports Medicine
210 East 64th Street
New York, NY 10065
E-mail: doctor_long@hotmail.com
Phone: (212) 434-4480
Fax: (212) 434-4313

 

Abstract:

The effect of introducing a femoral component that more closely resembles female native morphology, including a thinner anterior flange with an increased Q-angle and a decreased mediolateral (ML) to anteroposterior (AP) ratio (aspect ratio), was assessed with respect to the incidence of lateral patellar release and femoral component size. 79 consecutive total knee arthroplasties using a standard femoral component in women were compared with a subsequent consecutive series of 80 total knees performed with a modified design.  There was a statistically significant (p< 0.01) decrease in the rate of lateral release from 13 (16.5%) to 3 (4.3%) when the modified femoral component was used.  For each tibial size used, there was a trend towards using a larger femoral component with the modified design.

J.Orthopaedics 2008;5(4)e12

Keywords:

Lateral release; Female TKA; Modified femoral design.


Introduction:

The history of total knee arthroplasty (TKA) is one of constant innovation with progress towards components that more closely reproduce normal human knee dimensions and kinematics. Early components began with one, or few sizes. As the success rate and application of the procedure grew, multiple sizes were introduced to address the range of knees undergoing surgery [1].  

Femoral components were initially designed to be non-side specific. High rates of patellar complications, patellar clunk, and lateral release occurred. Further research resulted in side specific components with attention to the patellofemoral anatomy. This resulted in a decrease in failures, and an almost complete elimination of patellar clunk problems [8, 14]. Modified surgical techniques and prosthesis design, particularly with respect to component rotation and position [23, 25], have resulted in improved patellar tracking, reduced need for lateral release, and decreased patellofemoral specific modes of failure.  

Despite these changes, rates of lateral release continue to run as high as to 56.9 percent in combined studies of male and female patients [20], though in most series values range from 3-30 percent [2, 17-20, 26].  This procedure is not without risk, as previous authors have noted increased rates of avascular necrosis (AVN) and patella related failures following release [4]. In contrast, failure to address patellar tracking appropriately can result in increased rates of patellar and extensor mechanism complications [5, 6]. 

Modified trochlear dimensions and sizing options are a natural next step in femoral component design, as it has long been known that gender differences exist in native knee geometry and biomechanics [7, 12, 15, 16, 23, 27]. Clinical outcomes in females have also been inferior to those seen in men in some studies [3, 13]. This study was designed to look at the impact of this introduction on two related aspects of the surgical procedure: the rate of lateral patellar release needed for neutral patellar tracking and the size of the femoral component used.

Material and Methods :

Standard

The senior author  performed 79 consecutive primary TKAs using the Zimmer LPS flex system (Zimmer Inc., Warsaw, IN) in 51 females (14 bilateral) between September 2003 and June 2004. The average age was 64.9 years (range 45-84) and there were 46 left, and 33 right knees. BMI averaged 30.9 with a range from 18.6 to 53.3. 

Modified

The senior author performed 80 consecutive primary TKAs using a modified design (Zimmer LPS flex gender –Zimmer Inc., Warsaw, IN) in 69 females (9 simultaneous bilateral, 2 staged bilateral) between May 2006 and April 2007. The average age was 66.7 years (47-89) and there were 41 left and 39 right knees. BMI averaged 31.1 with a range from 19.5 to 52.1. 

Statistical analysis

Chi-squared test was used, with a p value 0.05 to determine significance. 

Technique

The senior author performed all TKAs. In all cases an incision only long enough to safely and accurately insert the components was used, and has previously been described at length [10, 11].  Femurs were placed using posterior rotational referencing, and tibial components using the medial 1/3 of the tubercle and crest of the tibia. 

With respect to the patella, a mini-medial parapatellar arthrotomy was performed in all cases. The patella was laterally subluxed, but not everted. The patellar preparation was performed following the femoral and tibial cuts, to allow more space for application of the patellar clamp. Patellar reaming was then performed to an appropriate depth to recreate the pre-patellar height with the addition of the button. Onlay circular patellar component size was chosen to provide maximum coverage without overhang.  Following application of the button, any lateral bone, and osteophyte were bevelled using a saw. 

Trial components were then inserted and the patellar tracking was observed using a ‘no thumbs’ technique [24]. The tourniquet was not released for this step. If lateral tracking was observed a towel clip was used to provide longitudinal tension to the extensor mechanism [10, 11], and tracking was reassessed as the ‘no thumbs technique’ has been shown to over represent the need for lateral release [2]. In cases where lateral tracking or tilt persisted, a lateral release was performed. A soft tissue sleeve of a minimum 1 centimetre was left lateral to the patella, and the superolateral vascular structures were identified and preserved.  In all cases, a lateral release provided central tracking. After cementing all components, a trial tibial insert was placed, and patellar tracking was once again assessed.
 

Results :

Standard Femoral Component

Standing anteroposterior x-rays were used in all cases for measurements of alignment. Pre-operative tibio-femoral alignment was a mean 0.5 degrees of varus (range 25 degrees of valgus to 14 degrees of varus). There were 5 patients with 15 or more degrees of valgus and 5 with 10 or more degrees of varus. Post-operative tibio-femoral alignment averaged 4.1 degrees of valgus (range 1 to 8 degrees of valgus).  

The average pre-cut patellar thickness was 22.0 mm and the average post-cut thickness, following cementation of the button, was 21.6 mm. In no cases was the difference greater than two millimetres for this measured value. In two cases (bilateral same patient) a patelloplasty was performed and the patella was not resurfaced due a very thin 15 mm preoperative thickness. In one other case a 24mm patella was left unresurfaced at the surgeon’s discretion in a young, non-obese patient with central tracking and minimal patellar chondral changes. None of the unresurfaced patellae required a lateral release.  

Table 1: Lateral release rate with standard femoral components in women

Femoral component size

C

D

E

F

Totals

no lateral release

2

23

33

8

66

lateral release

1

1

5

6

13

Totals

3

24

38

14

79

Table 2: Lateral release rate with modified femoral components in women

Femoral component size

C

D

E

F

Totals

no lateral release

0

23

35

19

77

lateral release

0

0

2

1

3

totals

0

23

37

20

80

Figure 1: Cases of lateral release and corresponding femoral size with the standard femoral component

In 13 (16.5 percent) cases a lateral release was required for central tracking of the patella (Table 1). The pre-operative alignment in these cases averaged 4.5 degrees of tibio-femoral valgus with 2 cases in 15 or more degrees of valgus. The rate of lateral release was higher in larger femoral components (Figure 1). In cases where an F (the largest size implanted) femoral component was used, six of fourteen knees required a lateral release. 

There was one complication in the lateral release group during the post-operative course. A postoperative lateral dislocation occurred in one case following a hyperflexion injury which resulted in a dehiscence of the medial arthrotomy. This necessitated repeat surgery at three months and again at 19 months post-operatively. Ongoing problems with patellar tracking have occurred, and a revision was performed. The patient is doing well with no recurrent patellar problems at early follow-up. 

Figure 2: Standard femoral component sizes used

Figure 3: Modified femoral component sizes used

 

Modified Femoral Component

Pre-operative standing tibio-femoral alignment was a mean 0.6 degrees of valgus (range 25 degrees of valgus to 18 degrees of varus). There were 11 patients with 15 or more degrees of valgus and 16 patients with 10 or more degrees of varus. Post-operative tibio-femoral alignment averaged 5.6 degrees of valgus (range 3 to 9 degrees of valgus). Alignment values were not statistically significantly different from the standard to the modified group. 

The patella was resurfaced in all cases in this group. The average pre-cut patellar thickness was 22.1 mm and the average post-cut thickness, following cementation of the button, was 22.3 mm. In no cases was the difference greater than two millimetres for this measured value. 

In three cases (4.3%) a lateral patellar release was required for central patellar tracking (Table 2). Two of these were in one patient undergoing bilateral TKAs with preoperative anatomic alignments of 10 and 15 degrees of valgus. The only other case requiring a lateral release was in a patient with 18 degrees of valgus. Femoral components (requiring lateral release) used included two E and one F component. 

Lateral Release Rate

In the 79 standard cases 13 (16.5%) required a lateral patellar release, versus 3 (4.3%) of modified femoral component cases. This difference was statistically significant (p< 0.01). 

Component Sizing

Femoral component size, when plotted versus tibial component size, shows a trend towards the use of larger femoral components with the modified design, than with the standard femoral component (Figures 2 and 3). This trend was not statistically significant.

Discussion :

Introduction of a modified femoral design was associated with a statistically significant reduction in the rate of lateral patellar release. There was a trend towards using larger femoral components with the modified design when standardized for corresponding tibial component used. The theoretical basis for design modifications in femoral components has been reviewed at length [9, 15], but this is the first study that we are aware of, that demonstrates a benefit to introducing a modified femoral design in females.  

The advantage to avoiding lateral patellar release is well known. Complications involve increased rates of AVN and patella related failures following release [4]. In contrast, failure to address patellar tracking appropriately can result in increased rates of patellar and extensor mechanism complications [5, 6]. 

There are three alterations that have been made in this new femoral design, all of which better approximate native female anatomy: the thickness of the anterior flange of the femoral component is reduced; the angle of the trochlear groove is increased by ten degrees, to better approximate the higher Q-angle in females; the M-L to A-P aspect ratios are reduced, thus decreasing overhang, allowing a more lateralized position for the femoral component, and maintaining posterior condylar offset. These modifications are consistent with anatomic gender differences [9, 15], and would also be presumed to decrease the lateral release rate based on a large outcome study by Pierson et al. showing that the odds ratio for lateral release increased 2.2 times for every centimetre increase in anterior femoral offset [22]. 

It is interesting to note that the need for a narrower femoral component is not a new concept. The Insall-Burnstein component had a special mid-size with a reduced ML to AP size ratio.  Poilvache et al. in their 1996 study on measured bone parameters about knees at the time of TKA concluded that ‘some narrow femora could require narrower implants to avoid medial-lateral overhang of the femoral component’ [23]. Other investigators have made similar observations with respect to ML to AP ratios in women versus men [6, 15], indicating the possible benefit to modified femoral designs. 

The rate of lateral patellar release was higher when a larger femoral component was used, particularly with the standard design (Table 1 and Figure 1). This is consistent with previously published anthropometric measurements, which showed that the discrepancy in aspect ratio between the native female anatomy and standard component sizes increased as femoral component size increased [15]. It is also consistent with a recently published large study of lateral release rates in total knee arthroplasty [22]. Overhang of larger components leads to stretching of the lateral tissues, likely leading to the higher rate of lateral patellar tracking and need for lateral release. If a smaller size is chosen to decrease overhang, a flexion extension mismatch is created. Though the trend towards more lateral releases with larger components also existed with the modified femoral component, it was not nearly as strong (Table 2).  

A second important benefit to a modified design is a more balanced flexion-extension gap. The ability to insert a larger femoral size decreases the risk of flexion instability as posterior condylar offset is maintained, and provides a more balanced tibio-femoral articulation. Clinical follow-up will be important to assess whether this results in a decrease in mid flexion, and flexion instability in this group.

Conclusion:

This study demonstrates a statistically significant decrease in the rate of lateral patellar release with the use of a modified femoral component in females. Despite this decrease, a small number of cases continued to require a lateral release, particularly in cases with significant pre-operative tibio-femoral valgus combined with a larger femoral component. A trend toward the use of larger modified femoral component sizes, when standardising for tibial size was also observed.  Further follow-up studies will be important in determining the significance of these operative changes on the long-term outcome of this new prosthetic design.

Reference :

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25. Scuderi GR, Insall JN. Rotational positioning of the femoral component in total knee arthroplasty. Am J Knee Surg. 2000 Summer;13(3):159-61.

26. Sodha S, Kim J, McGuire KJ, Lonner JH, Lotke PA. Lateral retinacular release as a function of femoral component rotation in total knee arthroplasty. J Arthroplasty. 2004 Jun;19(4):459-63.

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This is a peer reviewed paper 

Please cite as : William J. Long: The Impact of Component Design on Lateral Patellar Release Rates in Female Primary Total Knee Arthroplasty

J.Orthopaedics 2008;5(4)e12

URL: http://www.jortho.org/2008/5/4/e12

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