Abstract
Introduction
Persistent anterior knee pain, subluxation or dislocation of the patella as well as early aseptic loosening and increased polyethylene wear of the patella implant are common clinical problems after total knee arthroplasty (TKA) which are associated with the patellofemoral joint. In addition to patellar resurfacing, the design of the patellofemoral joint surfaces is attributed a large influence. While for patients without patella resurfacing, the native patella is sliding on the standardized femoral component and therefore the possibility of a reduced surface matching is high, patella resurfacing has been shown to decrease the joint contact area and yield to increased patellofemoral pressure. With regard to a further design optimization, the current study examined patellar biomechanics after TKA without and with resurfacing, comparing 5 differently designed patellofemoral joint surfaces of the femoral implant.
Methods
The femoral implant of the Genesis II prosthesis (Smith & Nephew) was scanned and an adaptable CAD-model was built using CATIA. Five different designs of the patellofemoral groove were created:
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original
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completely flat
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laterally elevated (+2mm lateral, −1mm medial)
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medially elevated (+2mm medial, −1mm lateral)
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laterally & medially elevated (+3mm lateral+medial)
The tibiofemoral joint as well as patellofemoral groove path and radius remained unchanged. Rapid Prototyping was used to produce prototypes made of polyamide.
A dynamic muscle loaded knee squat was simulated on 10 fresh frozen knee specimens with an upright knee simulator. The patellofemoral pressure distribution was measured using a flexible, resistive force sensor (TEKSCAN) while tibiofemoral and patellofemoral kinematics were recorded with an ultrasonic motion tracking system (ZEBRIS). In addition, patellar stability was measured in different flexion angles on another 10 specimens using a robot (KUKA). Measurements were taken on the native knee as well as after TKA and after additional patellar resurfacing with alternating femoral implant.
Results
Considering patellofemoral kinematics, the largest influence was found for the flat design where increased lateral tilt (up to 6°) and medial shift (up to 5mm) were measured after TKA compared to the native knee. Similar results were achieved for patellar stability, where increased lateral displacement (up to +6mm) was measured for the flat design. The other designs only had a small effect on patellar kinematics and patellar lateral stability. Regarding maximal peak pressure, on average, only a small influence of the designs was found. However, for the individual knee specimens, the pressure distribution and peak pressures varied clearly among the different designs.
After additional patellar resurfacing, patellofemoral peak pressures significantly increased (almost doubled), but for the different designs, similar results as before were achieved regarding patellar kinematics, stability and pressures.
Conclusions
Increased mediolateral motion was found for the flat design compared to the others and the native knee concluding that a moderate groove is necessary but also sufficient to guarantee stable motion. Especially for maximal patellofemoral peak pressures, large individual differences between the designs were measured while the average influence was small. Therefore, an individual choice between some standard implants might be an interesting option, if appropriate criteria can be found.
