PSEUDO-PATELLA BAJA FOLLOWING SOFT TISSUE BALANCING IN TOTAL KNEE ARTHROPLASTY

Abstract

Introduction: Patella baja is the distal positioning of the patella in relation to the femoral condyles in the sagittal plane. True patella baja is due to shortening of the patella tendon (PT), as measured by the Insall-Salvati method, and narrowing of the distance between the patella and the proximal tibia. Pseudo-patella baja describes narrowing of the distance between the patella and the proximal tibia without shortening of the PT and occurs following Total Knee Arthroplasty (TKA), where the tibial prosthesis plus insert are thicker than the resected tibia. Both may cause patella pathology, pain and a reduced range of motion. Pseudo-patella baja can be detected using the Caton-Deschamps method.

Soft tissue balancing is an important factor in the success outcome of TKA, but if extensive can necessitate the use of thicker tibial inserts. This may alter the position of the patella in relation to the tibia and increases the risk of creating a pseudo-patella baja.

Hypothesis: Patients who undergo extensive soft tissue releases during TKA, with the resultant use of thicker tibial inserts will develop a pseudo-patella baja, and will have a poorer outcome and reduced range of motion.

Method: In this prospective study, 506 patients aged 40–90 years underwent 526 Kinemax TKAs, performed by 7 surgeons in 5 centers between October 1999 and December 2002. The extent of soft tissue releases and the size of tibial inserts were recorded. Independent observers used the Caton-Deshamps method to assess patella position measured Pre and post-operative lateral radiographs. The patients were also assessed using the Oxford Knee Score and the American Knee Society Clinical Rating System, with a minimum follow-up of 12 months.

Results:

1. TKA surgery creates a Pseudo-Patella Baja. Excluding patients with a pre-operative patella baja, pseudo patella baja was introduced into 25.6% of patients. (p=0.00).

2. Extensive soft tissue releases during TKA are associated with a 100% increased in the incidence of pseudo patella baja compared to more moderate soft tissue releases. (p=0.002).

3. The use of large tibial inserts is associated with a significant increase in the incidence of pseudo-patella-baja, compared to smaller inserts. Three groups were identified: Small Inserts 8 mm, Medium inserts 10–12mm, and Large inserts 15, 18 & 22 mm. (p=0.042).

4. There was no correlation between the incidence of a pseudo-patella baja and changes in clinical or functional outcome, including range of motion, as measured using the OKS and AKSCRS.

Conclusion: Pseudo patella baja occurs in 25% of all patients following TKA, and in over 40% of patients in whom extensive soft tissue releases have been performed and/or large tibial inserts have been used. At 12 months, no detrimental outcomes were attributable to the incidence of pseudo patella baja.