Patellar tendon length during knee flexion of meniscal-bearing and rotating total knee arthroplasty implants.

PURPOSE: The length of the patellar tendon after total knee arthroplasty (TKA) has most commonly been compared with preoperative measurements. However, there are no reports that discuss changes in the length of the patellar tendon during knee flexion after TKA. The purpose of this study was to evaluate changes in the length of the patellar tendon during knee flexion and to clarify the impact of changes in patellar tendon strain on the length of the patellar tendon and post-operative range of motion (ROM) after TKA.

METHODS: Thirty-six patients undergoing sequential bilateral TKA for osteoarthritis were evaluated. Patients received a meniscal-bearing (MB) implant on one side and a rotating platform (RP) implant on the other and were followed for a median of 115 months (range 60-211 months). The lengths of the patellar tendon at maximum extension, 30°, 60°, 90° and maximum flexion were measured, and the post-operative ROM of both knees were assessed. The effects of implant design and the knee flexion angle on the length of the patellar tendon were analysed using a linear mixed-effects model. The relationship between patellar tendon strain and post-operative knee ROM was assessed using Pearson's correlation coefficients.

RESULTS: Post-operative clinical scores were similar for MB and RP implants. Neither the implant design nor the knee flexion angle significantly affected the length of the patellar tendon. The ROMs in the two designs at final follow-up were equivocal (114° in MB, 113° in RP). There was no correlation between patellar tendon strain and ROM in knees with either implant type.

CONCLUSIONS: Patellar tendon lengths after mobile-bearing TKA with implants that permitted different anteroposterior constraints were relatively constant at varying degrees of knee flexion. Differences in patellar tendon strain may not impact ROM. These results provide conclusive evidence that the quality of the patellar tendon may play a less important role in ROM after TKA.

LEVEL OF EVIDENCE: Therapeutic study, Level II.