Secondary Patellar Resurfacing in TKA: A Combined Analysis of Registry Data and Biomechanical Testing

The German Arthroplasty registry (EPRD) has shown that different prosthesis systems have different rates of secondary patellar resurfacing: four years after implantation, the posterior-stabilized (PS) Vega prosthesis has a 3.2% risk of secondary patellar resurfacing compared to the cruciate-retaining (CR) Columbus prosthesis at 1.0% (both Aesculap AG, Tuttlingen, Germany). We hypothesized that PS implants have increased retropatellar pressure and a decreased retropatellar contact area compared to a CR design, which may lead to an increased likelihood of secondary patellar resurfacing. Eight fresh frozen specimens (cohort 1) were tested with an established knee rig. In addition, a possible influence of the registry-based patient collective (cohort 2) was investigated. No significant differences were found in patient data–cohort 2-(sex, age). A generally lower number of PS system cases is noteworthy. No significant increased patella pressure could be detected with the PS design, but a lower contact area was observed (cohort 1). Lower quadriceps force (100°–130° flexion), increased anterior movement of the tibia (rollback), greater external tilt of the patella, and increasing facet pressure in the Vega PS design indicate a multifactorial cause for a higher rate of secondary resurfacing which was found in the EPRD patient cohort and might be related to the PS’ principle function.

Outcome Validation of a Simulation Based Patient Specific TKA Planning Tool

Dynamic knee computer simulations are a promising surgical planning option in TKA, allowing the impact of plan alterations on joint dynamics to be analysed prior to surgery. Previously, the dynamic results of our simulation have been shown to correlate with outcome; here we show validation of its use in pre-operative planning.A database of TKA Patients undergoing surgery from 1-Jan-2014 operated on by 9 surgeons, who received a pre-operative and post-operative CT were assessed. A musculoskeletal computational model with similar boundary conditions to the Oxford Knee Rig was used to simulate post-TKA knee dynamics using Adams MSC software (Newport, CA). In addition, a set of pre-operative simulations were generated covering positional variations. The Dynamic Knee Score (DKS), a predictive algorithm machine learned from KOOS scored postoperative cases to predict outcome in preoperative planning was applied to all simulations.Patients were split into groups depending on whether the ‘post-operative achieved position’ was the ‘best’ of the preoperative modelled options in terms of simulated DKS score or not. These results were compared with 12 month postoperative KOOS scores. Cases where the best plan was followed had better outcome results. A relationship was shown with the KOOS Pain subscore, with the portion of patients below a KOOS Pain score of 70 dropping to 11% from 16% (p=0.030) when the best plan was followed.This study shows significant relationships between selection of patient specific kinematically optimal surgical plan and outcome. Such tools will play an important role in future patient specific decision making.

Kinematics of the knee joint

In 2006 a test rig was built by Ghent University to measure patellofemoral pressures. The maingoal of the current study is to improve and upgrade the Ghent Knee Rig (GKR). The improvement consistsof a better approach of the Q angle. The mechanical connection between the linear actuator and the rectusfemoris tendon represents the force of the quadriceps. To validate the results of measurement, it isnecessary the connection approaches the direction of the real muscle. The patella is bond with the body byuse of ligaments and retinacula, which have a unknown influence on the stability of the patella. Alsoquadriceps and the trochlea are important stabilizers. The upgrade of the GKR is a tool to measure thepatella stability, which is done by pulling the patella out of the trochlea and measure the force needed for acertain displacement. It is important the patella has five degrees of freedom (DOF) so no extra force but thestability is measured and the effect of dysplasia and trochleoplasty can be examined. In the near future theGKR will be expanded to the possibility of measuring patella tracking. The relative movements of thedifferent components of the knee will be followed by high speed cameras.