In vitro measurement of patellofemoral force after three types of knee replacement

Abstract

U sing a new, non-invasive method, we measured the patellofemoral force (PFF) in cadaver knees mounted in a rig to simulate weight-bearing. The PFF was measured from 20° to 120° of flexion before and after implanting three designs of knee prosthesis.Medial unicompartmental arthroplasty with a meniscal-bearing prosthesis and with retention of both cruciate ligaments caused no significant change in the PFF. After arthroplasty with a posterior-cruciateretaining prosthesis and division of the anterior cruciate ligament, the PFF decreased in extension and increased by 20% in flexion. Implantation of a posterior stabilised prosthesis and division of both cruciate ligaments produced a decrease in the PFF in extension but maintained normal load in flexion.There was a direct relationship between the PFF and the angle made with the patellar tendon and the long axis of the tibia. The abnormalities of the patellar tendon angle which resulted from implantation of the two total prostheses explain the observed changes in the PFF and show how the mechanics of the patellofemoral joint depend upon the kinematics of the tibiofemoral articulation. [Br] 1998;80-B:900-6. J Bone Joint Surg Received 16 October 1997; Accepted after revision 4 June 1998Patellofemoral problems are a common cause of failure after total knee replacement (TKR). Patellar resurfacing implants have often shown loosening or wear of their polyethylene surfaces 1,2 and large increases in anterior patellar strain have been reported after TKR, 3,4 suggesting that joint replacement may have an adverse effect on the mechanics of the extensor mechanism of the knee. We have devised a non-invasive method for measuring the patellofemoral force in a cadaver knee subjected to lifelike loads in a laboratory rig. 5 We now describe the use of that technique to measure changes in the magnitude of the patellofemoral force after the implantation of three designs of knee prosthesis into cadaver joints. Materials and Methods Non-invasive measurement of the patellofemoral force.Three forces act on the patella in the sagittal plane (Fig. 1a). The quadriceps tendon force (QTF) and the patellar tendon force (PTF) are tensile; the patellofemoral force (PFF) is compressive. Equilibrium requires that, if only these three forces act on the patella, they must be coplanar and concurrent at the centre of force of the patella (P). In principle, our method of measurement of the PFF is to replace it with a measured tensile force (TF), applied at point P through a cable attached to the anterior aspect of the bone (Fig. 1b). The tension in the cable is increased until patellar lift-off occurs, at which point TF is deemed to have replaced PFF exactly (Fig. 1c). A detailed description of the testing rig, the method of determining the centre of pressure of the patella and the direction and magnitude of PFF have been published previously. 5Experimental procedure. Eight fresh human cadaver knees were taken at post mortem and quick frozen to -20°C. Immediately before testing each knee was thawed to ro...

In‐vivo patellar tendon kinematics during weight‐bearing deep knee flexion

“…Therefore, any simulation of the patellar tendon forces must consider the inhomogeneous deformation of the tendon at deep flexion, such as using those using robotic technology 31 or Oxford rigs. 32 These data may provide insight into knee function in deep flexion when designing surgical techniques that are aimed to restore function in deep flexion.…”

How effective are added constraints in improving TKR kinematics?

“…As well as using the motion of the contact points to quantify in vivo tibiofemoral kinematics, we also assessed the overall knee kinematics using the PTA/KFA relationship or kinematic profile. The PTA provides a useful global picture of the mechanical function of the knee as it is influenced by tibial-femoral and patellar-femoral movement (Gill and O'Connor, 1996;Miller et al, 1998;Price et al, 2004;Rees et al, 2005). There are distinct advantages of analysing PTA/KFA and cam-post interaction together with the analysis of the 3D contact points.…”

Improved quadriceps' mechanical advantage in single radius TKRs is not due to an increased patellar tendon moment arm

“…However, the effect of TKR surgery on this transmission has not been thoroughly investigated. Since TKR surgery is known to significantly alter the geometry of the patellar mechanism [17,18], it is likely that force transmission changes between the patellar and quadriceps tendons, thereby altering QF:PTF and influencing quadriceps' mechanical advantage.…”