Normal knee function following total knee replacement (TKR) is determined by an appropriate balance of joint laxity and stability. TKR is increasingly being performed on patients 55 years of age and younger, and these patients are more likely to have post-operative complications than older patients. It has been speculated that younger patients place greater functional demand on their replaced joint sometimes exceeding the capabilities of the implants. The impetus behind this dissertation was the creation of novel methodologies, both computational and experimental, that could be used to systematically investigate the dynamic relationship between implant design and function. Four studies investigated the relationship between implant design or surgical technique and joint stability and function.

The first study involved the development of a dynamic computer simulation of a TKR implant range of constraint test. A rigid-body-spring model was utilized to compute implant contact forces and experimental measurements were used to validate the model. Model outputs compared favorably to experimental constraint values only when artifacts in the laxity testing simulator were represented.

The second study augmented the range of constraint test simulation by modeling the posterior cruciate ligament (PCL). The addition of the PCL made it possible to assess laxity in TKR designs that retain this ligament. A tight PCL can limit joint laxity and impair function following TKR. Two surgical treatment options, increased posterior tibial sloping and partial PCL release, were simulated to assess their individual and combined effectiveness at reducing PCL tension and improving laxity. Although both treatments were effective at increasing anterior laxity, only partial PCL release was effective at increasing overall anterior-posterior laxity.

The third study employed a novel dynamic mechanical TKR simulator to assess functional differences between two variations of a reduced patellar height. Reduced patellar height is likely to disrupt the extensor mechanism following TKR. The mechanical knee simulator applied physiological loads across both the tibiofemoral and patellofemoral joints. Both forms of reduced patellar height substantially increased the quadriceps force required to extend the knee. With a reduced patellar height, a symmetric patellar implant was more resistant to abnormal tracking than an anatomic patellar implant.

The final study investigated the micromotion of two cementless femoral components with different designs during squat, gait and chair rise simulations. The presence of micromotion above a certain threshold can prevent bony ingrowth and potentially lead to implant loosening. A novel method was developed to measure the pattern of micromotion between the femoral component and an underlying bone substitute. No statistical differences in micromotion were noted between the TKR designs for gait or chair rise simulations, but significant differences were found for squatting simulations. The micromotion of the femoral component was found to occur in three dimensions, contrary to previous reports.

In conclusion, this dissertation outlined novel methodologies used to investigate the role of TKR motion. Through these research methodologies new TKR designs can be evaluated and improved prior to implantation. Although TKR is an attractive and effective approach for thousands of patients afflicted with arthritis, future work should be directed at optimizing TKR design and surgical technique to further improve postoperative function.