Introduction

The metal-backed patella was originally designed to address shortcomings found with cemented, all-polyethylene patellae. However, complications relating to an all-polyethylene patella were reported to account for up to half of all knee revisions. At the same time, good fixation with bone ingrowth was observed in both titanium and cobalt chromium porous-coated patellae. The advantages provided by using a metal-backed patella, such as uniform load sharing, decreased polyethylene deformation, and potential for biological fixation, may be unjustly outweighed by the fear of patellar component failure; high rates of failure have not been inherent to all metal-backed patella designs. Over the past decade, we have used a metal-backed patella design with excellent results that may be due largely to the design features of the component. Also, we believe there are certain selection criteria that should be strictly adhered to when implanting metal-backed patellae. Correct selection criteria and improved component design strongly indicate the use of press-fit metal-backed patellae.

Introduction:

For 30 years, uncemented anatomic hip stems have been implanted with documented clinical results[1,2]. Their geometry can be linked back to the geometry of the PCA and ABG stems. Modifications to date include stem length, body geometry, material, and reduction in distal geometry. New tools have been developed allowing anatomical measurements and analysis of three-dimensional digital femora geometry through CT scans[3]. The purpose of this study is to analyze three-dimensional contact of various anatomic hip stem designs using this technique.

Total knee arthroplasty (TKA) increasingly is utilized to treat younger, more physically active, or more culturally diverse patients who desire the ability to perform activities with high knee flexion. As a result, many implant manufacturers have modified designs or introduced new ones to better facilitate deep knee flexion. To date, a mix of studies has reported superior or equivalent flexion performance comparing high-flexion and traditional implant designs. Importantly, many of these studies are conducted with the patient supine in non-weightbearing postures, not in functional postures where differences in joint mechanics are better manifest. The goal of this study was to evaluate weightbearing kneeling and lunging knee kinematics in patients with bilateral TKA of two types.

Nine high functioning patients from the American Southwest provided informed consent to participate in this single-surgeon study. The subjects averaged 74 years of age and included three females. Each subject received a traditional cruciate-retaining TKA in one knee and a flexion-enhanced cruciate-retaining (7 knees) or posterior-stabilized (2 knees) TKA in the other. The traditional knees were an average of 84 months postoperative and had combined Knee Society Scores averaging 183. The knees with new TKA designs were an average of 31 months postoperative and had combined Knee Society scores averaging 188. Subjects were observed performing a weight-bearing lunge to maximum comfortable flexion and partially weightbearing kneeling to maximum comfortable flexion using lateral fluoroscopy. Model-image registration techniques were used to quantify the 3D translations and rotations of the tibial and femoral components.

There were no differences in maximum knee flexion during lunging (115°±12° versus 118°±7°) or kneeling (120°±14° versus 120°±10°) for the traditional and flex-ionenhanced TKA’s. Tibial internal rotation and abduction were not different. The locations of the medial and lateral condyles were significantly more posterior in the traditional design for both activities (p< 0.05).

This study examined maximum flexion knee kinematics in clinically excellent, high performing subjects with bilateral TKA of two types. No clinically important functional differences were observed. Although flexion-enhanced designs may provide improved flexion for patients who demand it, older patients living a Western lifestyle appear to do equally well with the traditional and flexion-enhanced TKA designs.

Range of motion after total knee arthroplasty (TKA) is increasingly an issue of critical concern in the performance of knee replacements and their ability to meet the functional demands of diverse patient populations. The goal of this study was to measure maximum weight bearing flexion in patients with one knee replaced using a posterior cruciate retaining (CR) device and the other a cruciate substituting (PS) device on a prospective basis.

Sixteen patients (32 knees) with bilateral TKA and excellent clinical/functional outcomes at least one year after surgery participated in this study. Eight patients received knee replacements from one surgeon, and eight patients received knee replacements of another design from a second surgeon. Their knees were imaged using fluoroscopy as the patients were asked to bend to maximum flexion with their foot on a 25 cm step. CAD model based shape matching techniques were used to measure knee flexion, axial rotation, and the anterior/ posterior (AP) location of tibio-femoral contact relative to the AP centre of the tibial insert. Paired t-tests were used to evaluate differences in means (p = 0.05).

PS knees had larger average flexion angles (121±8 deg vs. 114±5 deg, p=0.01) and greater femoral rollback (14±3 mm vs. 12±3 mm, p=0.02) than the CR knees. There was no difference in axial rotation (6±4 deg vs. 5±6 deg). The same trends were observed within the single design subgroups.

In patients with heterogeneous bilateral TKA, greater femoral rollback and weight-bearing flexion is exhibited by the knee with a posterior stabilised TKA.