Abstract
INTRODUCTION
Telemetric implants have provided us with invaluable data as to the in vivo forces occurring in implanted knee joints. However, only a few of them exists. The knee is one of the most studied joints in the human body and various mathematical knee models have been used in the past to predict forces. However, these simulation studies have also been carried out on a small group of patients limiting their general usefulness in understanding overall trends of knee behavior. Therefore, it is the purpose of this research to study the implant forces experienced by a large group of patients so as to have a better understanding of the overall magnitudes and their variability with knee flexion.
METHODS
The patients were selected from a large database of over 3000 knees for which kinematic analysis had previously been carried out using fluoroscopy. The criteria used for selection was that the patients had a successful knee implant (HSS >90) and were able to perform a weight bearing deep knee bend of at least 110 degrees. The patients were randomly chosen without any other restrictions. The kinetic analysis was carried on a cohort of over 100 patients using a previously published inverse dynamic rigid body model. This model, which has been validated using telemetric data, is capable of predicting the contact forces on the medial and lateral condyles of the knee. Analysis was carried out till 130 degrees of flexion to remove any effect of thigh calf contact that the model does not incorporate. 20 normal knees were also included for comparison.
RESULTS
The contact force variation through the weight bearing flexion cycle can be divided into 3 distinct regions: (1) 0 degrees to 90 degrees the contact forces generally increase, (2) 90 to 120 degrees the forces reach a peak (3) beyond 120 degrees the forces decrease (Figure 1). Though similar at full extension, the contact forces in the implanted patients were found to be significantly higher than the normal knee subjects with increase in flexion. The maximum force for any implanted patient was found to be around 4.5BW while for the normal knee it was around 3.3BW. For the implanted knees the medio-lateral force distribution was close to 50-50 at full extension and increased slightly with the increase in flexion. For the normal knees however, the increase in flexion was found to increase the load on the medial side considerably (Figure 2).
DISCUSSION
From a design perspective, it is good to note that the contact forces reduce at higher flexion angles. This is probably due to the wrapping of the quadriceps tendon on the femur causing the moment arm of the quadriceps to increase as the contact points move posterior with flexion. Compared to the normal knee, contact positions in implants tend to stay more anterior which causes higher contact forces. Ligament release, performed during knee implant surgery, tends to equal out the medio-lateral force distribution as compared to the normal knees.
