Is There an Ideal Patellar Thickness Following Total Knee Arthroplasty?

Comparison of Clinical Results and Risk of Patellar Injury between Attune and PFC Sigma Knee Systems

Purpose

The purposes of this study were to compare clinical results after total knee arthroplasty (TKA) using the Attune and PFC Sigma knee designs and to investigate whether the use of the Attune prosthesis increased the risk of patellar injury in Asian patients.

Materials and Methods

Three hundred knees that underwent TKA using Attune (group A) were compared to 300 knees that underwent TKA using PFC Sigma (group B). The Knee Society Knee Score (KS) and Function Score (FS), and range of motion (ROM) were compared. The residual patellar thickness was compared to evaluate the risk of patellar injury.

Results

The postoperative KS and ROM of group A were better than those of group B (93.1 vs. 88.8, p<0.001 and 131.4° vs. 129.0°, p=0.008, respectively). The postoperative FS did not differ significantly between the two groups (80.9 vs. 78.7, p=0.427). The residual patella was thinner in group A (14.8 mm vs. 15.7 mm, p=0.003), which made up a higher proportion of the high-risk group for patellar fractures with a residual thickness of <12 mm (7.5% vs. 2.1%, p=0.003).

Conclusions

TKA using the Attune prosthesis provided more favorable clinical results than TKA using PFC Sigma. However, the risk of injury in the residual patella was increased with use of the Attune prosthesis in Asian patients.

Kinematic alignment of current TKA implants does not restore the native trochlear anatomy

INTRODUCTION

Preserving constitutional patellofemoral anatomy, and thus producing physiological patellofemoral kinematics, could prevent patellofemoral complications and improve clinical outcomes after kinematically aligned TKA (KA TKA). Our study aims 1) to compare the native and prosthetic trochleae (planned or implanted), and 2) to estimate the safety of implanting a larger Persona^® femoral component size matching the proximal lateral trochlea facet height (flange area) in order to reduce the native articular surfaces understuffing generated by the prosthetic KA trochlea.

METHODS

Persona^® femoral component 3D model was virtually kinematically aligned on 3D bone-cartilage models of healthy knees by using a conventional KA technique (group 1, 36models, planned KA TKA) or an alternative KA technique (AT KA TKA) aiming to match the proximal (flange area) lateral facet height (10 models, planned AT KA TKA). Also, 13postoperative bone-implant (KA Persona^®) models were co-registered to the same coordinate geometry as their preoperative bone-cartilage models (group 2implanted KA TKA). In-house analysis software was used to compare native and prosthetic trochlea articular surfaces and medio-lateral implant overhangs for every group.

RESULTS

The planned and performed prosthetic trochleae were similar and valgus oriented (6.1 and 8.5, respectively), substantially proximally understuffed compared to the native trochlea. The AT KA TKAs shows a high rate of native trochlea surface overstuffing (70%, 90%, and 100% for lateral facet, groove, medial facet) and mediolateral implant overhang (60%). There was no overstuffing with conventional KA TKAs having their anterior femoral cut flush.

CONCLUSION

We found that with both the planned and implanted femoral components, the KA Persona^® trochlea was more valgus oriented and understuffed compared to the native trochlear anatomy. In addition, restoring the lateral trochlea facet height by increasing the femoral component size generated a high rate of trochlea overstuffing and mediolateral implant overhang. While restoring a native trochlea with KA TKA is not possible, the clinical impact of this is low, especially on PF complications. In current practice it is better to undersize the implants even if it does not restore the native anatomy. Longer follow-up is needed for KA TKAs performed with current implant, and the debate of developing new, more anatomic, implants specifically designed for KA technique is now opened.

LEVEL OF EVIDENCE

II, Laboratory controlled study.

Safety and Efficacy of 6.2 mm Patellar Button in Resurfacing Less than 20 mm Thin Patella: A Matched Pair Analysis

Purpose

Restoring the native patellar thickness after patellar resurfacing provides optimal function of the knee after arthroplasty and minimises complications related to the patellofemoral articulation. The aim of this study was to assess the usefulness of a thin patellar button (6.2 mm) in patients with a patella thickness of less than 20 mm during total knee arthroplasty.

Materials and Methods

This is a retrospective case control study. A total of 54 female patients with an intraoperative patellar thickness of <20 mm, resurfaced with a patellar button of 6.2 mm in thickness were identified (group 1). They were matched with 54 patients with a patellar thickness of 20–23 mm, resurfaced with a patellar button of 8 mm (group 2), based on age, sex, body mass index, and deformity. A clinical and radiological evaluation was done at a minimum 2-year follow-up.

Results

The preoperative mean patellar thickness was 18.94±1.07 mm and was restored to 19.06±0.79 mm in group 1, as compared to 21.63±0.99 mm and 21.72±0.99 mm in group 2. The mean postoperative range of motion was 122.22°±9.25° in group 1 and 123.52°±8.72° in group 2 (p=0.13). No patellar bone or button related complications were observed in any patient in either group.

Conclusions

The 6.2 mm thin patella is useful to restore the native thickness in patients with a patellar thickness of less than 20 mm without risk of button fracture, loosening or overstuffing.

Patellar Cut and Composite Thickness: The Influence on Postoperative Motion and Complications in Total Knee Arthroplasty

BACKGROUND

Little data exist on the influence of patellar thickness on postoperative motion or complications after total knee arthroplasty (TKA). This study addresses the following questions: Is postoperative motion influenced by change in composite patellar thickness? Is change in patellar thickness associated with more complications? And do more complications occur in the knees with a patellar bone remnant (<12 mm) and a native patellar thickness <18 mm?

METHODS

In total, 3655 TKAs were performed by 3 surgeons over a 28-year interval. All knees had caliper measurement of patellar thickness before the patellar cut, after implantation of the component and postoperative motion recorded in the database 1 or 2 years after TKA.

RESULTS

Patellar composite thickness was the same (1034 knees), thicker (1617 knees), and thinner (1004 knees). A significant but weak relationship was identified between the change in patellar thickness and motion (P < .01, ρ = -0.046); an increase in "composite patellar thickness" of 10 mm would result in a 3° loss of knee motion. Significant differences were identified between change in thickness and manipulations (P < .05), ruptures (P = .01), and patellar clunk/crepitus (P < .01). Examining knees with bone remnant thicknesses (<12 mm/≥12 mm), there was no difference in fractures (P = .26). No extensor ruptures occurred in knees with remnant thickness <12 mm. Comparing knees with native bone thickness (≤18 mm/>18 mm), significant differences were found in fractures (P < .01) and patellar radiolucencies (P = .01).

CONCLUSION

As this data does not demonstrate a strong tendency toward losing motion when the patellar thickness is increased, the authors recommend avoiding compromise of the patellar bone stock and tendon insertion. When native patellar bone is thin (<18 mm), we recommend maintaining 12 mm of patellar bone stock and accept the increase in composite thickness.