Introduction

Unicompartmental knee arthroplasty (UKA) currently experiences increased popularity. It is usually assumed that UKA shows kinematic features closer to the natural knee than total knee arthroplasty (TKA). Especially in younger patients more natural knee function and faster recovery have helped to increase the popularity of UKA. Another leading reason for the popularity of UKA is the ability to preserve the remaining healthy tissues in the knee, which is not always possible in TKA. Many biomechanical questions remain, however, with respect to this type of replacement.

25% of knees with medial compartment osteoarthritis also have a deficient anterior cruciate ligament [1]. In current clinical practice, medial UKA would be contraindicated in these patients. Our hypothesis is that kinematics after UKA in combination with ACL reconstruction should allow to restore joint function close to the native knee joint. This is clinically relevant, because functional benefits for medial UKA should especially be attractive to the young and active patient.

Introduction

Patellar tracking in total knee replacements has been extensively studied, but little is known about patellar tracking in isolated patellofemoral replacements. We compared patellar tracking and the position of the patellar groove in the natural knee, followed by implantation of the femoral component of a PFR (patella unresurfaced) and after implantation of the femoral & patellar component of the PFR.

Introduction

Mobile-bearing TKRs allow some axial rotation and may provide a more natural patellar movement. The aim was to compare patellar kinematics among the normal knee, fixed-bearing and mobile-bearing TKR.

Introduction: Mobile bearing TKRs may allow some axial rotation and also compensate for a slight tibiofemoral rotational mismatch. This is thought to provide better kinematics and a more natural patellar movement. This theoretical advantage has not been verified in clinical studies for the tibiofemoral kinematics. However, little is known about the patellofemoral kinematics of mobile bearing TKRs. The aim was to compare patellar kinematics among the anatomic knee, fixed bearing TKR and mobile bearing TKR.

Methods: Optical computer navigation marker arrays (Brainlab) were attached to the femur, tibia and patella of 9 whole lower extremities (5 fresh cadavers). The trial components of a fixed bearing posterior stabilised TKR (FB) (Sigma PFC, Depuy) were implanted using a tibia first technique. Then the tibia component was changed to a posterior stabilised mobile bearing tibia component (MB) (Sigma RP Stabilised). The patellae were not resurfaced. The knees were moved through a cycle of flexion and extension on a CPM machine. Medial/lateral shift and tilt was measured relative to the patella position in the natural knee at full extension always with soft tissue closure. The path of the trochlea and patellar groove of the femoral component was registered. Values are expressed as mean+/−one standard deviation. Statistical analysis: two tailed paired Student’s T-test.

Results: M/L shift: There was a tendency for the patella to track 2mm more laterally throughout the flexion range with a FB or MB TKR compared to the natural knee, but this did not reach significance.

Tilt: The patella in the natural knee tilted progressively laterally from extension to flexion, plateauing at 50° of flexion (20°: 1.9+/−2.7°, 40°: 5.6+/−5.4°, 60°: 6.2+/−6.4°, 80°:6.5+/−7.3°, 90°: 6.4+/−7.7°). With a FB or MB TKR the patellae also tilted laterally up to 50 degree of flexion, but then started to tilt back medially, reaching the neutral position again at 90°. The patellae of the FB and MB TKRs were significantly more medially tilted at 50° to 90° of flexion compared to the natural knee. But there was no difference between the FB and MB TKRs. (Fixed bearing: 20°: 2.5+/−7.2° p=0.30, 40°: 3.7°+/−6.5° p=0.15, 60°: 3.1+/−5.8° p=0.02, 80°:1.2+/−6.5° p=0.001, 90°: 0.3+/−7.2° p=0.001, Mobile bearing: 20°: 0.3+/−5.5° p=0.27, 40°: 3.6+/−5.2° p=0.08, 60°: 2.1°+/−5.8 p=0.01, 80°: 0.2+/−6.8 p=0.003, 90°: −0.6+/−7.3 p=0.002; vs. natural)

Trochlea position: The centre of the patellar groove of the femur component was more lateral than the trochlea by 2–5mm, it also extended 10mm further proximally.

Conclusion: There are kinematic differences in patellar tracking between the natural and a FB/MB TKR. This may be due to a slightly different position of the patellar groove. The patellar kinematics of the MB TKR is not more natural compared to the FB TKR.

Introduction: Since the introduction of the Zimmer Innex UCOR (Ultra COgruent Rotating) mobile bearing total knee arthroplasty (TKA) system in 1999, there were close to 3000 primary TKAs performed at our institution utilizing this implant. We report on the first 396 5-year follow-up results and overall revisions in our total collective.

Methods: Between 1999 and 2006 there were 2734 primary Innex UCOR TKA performed (1748 female/987 male) at the Schulthess Clinic, Zurich. Primary diagnosis leading to TKA were OA (2462 – 90%), RA (144 – 5.3%), posttraumatic arthritis (65, 2.4%), necrosis (50, 1.8%) and misc causes (13 – 0.5%). The mean age of the females patients was 69y (33y – 92y), and 68y (31y – 93y) in the male population. To date 396 knees underwent clinical and radiological follow-up at 5 years (mean 5y 0m, range 4y 1m – 7y 2m), with 5% of the patients being lost to follow-up. Scoring was done, using the Knee Society Score (KSS). All patients had a full leg radiograph pre-operatively as well as at follow-up.

Results: Total KSS improved from 106.5 (6 – 184) pre-operatively to 179.5 (80 – 200) at follow-up, the knee score from 42.2 (2–93) to 92.1 (37 – 100), the function score from 64.3 (0–100) to 87.4 (10–100) respectively. The pain score increased from 17.7 (0–50) to 47.5 (20–50, 50 points maximum). ROM pre-operatively was 104.6° (0–145) and reached 117.2° at follow-up (55–145). Subjective evaluation by the patient at 5 years was excellent and good in 91%, fair in 8% and taxed poor by 1% of the patients. 95% of full leg radiographs showed a femorotibial angle of 182°–188°, 3% were < 182° (varus), 2% > 188° (valgus). Overall revision rate (95 of 2735) was 3.5%, 1.1% for infections and 1% for anterior knee pain and/or patella pathology. 0.6% were revised for instability, 0.3% for arthrofibrosis, and the remaining 0.5% for various problems.

Conclusion: These promising 5 year observations with the Innex UCOR mobile bearing TKA system suggest favorable overall midterm results. Further longer term follow-up evaluations are scheduled while 5 year follow-ups are ongoing, allowing for continuing reports on long-term performance.