BIOMECHANICS LAB

TREATMENT ALGORITHM FOR CHRONIC KNEECAP INSTABILITY AND THE INFLUENCE OF BODY MASS INDEX (BMI) ON CLINICAL OUTCOMES

Head of Research: Dr. Jan Herman Kuiper – Orthopaedic Interventions
Collaborators: Mr Andrew Barnett, Dr Kelly Campbell, Ms Taya Chapman, Dr Caroline Dover, Mr Pete Gallacher, Dr Shailesh Naire, Mr Simon Pickard, Dr Nikhil Sharma

Normally, the kneecap (patella) glides through a groove at the end of the femur (thighbone). Patients have a chronic patellar instability if their kneecap repeatedly moves outside the groove completely (dislocation) or partly (subluxation). Patients with a dislocation or subluxation have pain, swelling and difficulties moving the knee.

Patients with chronic kneecap instability can be helped using surgery, but this must be planned carefully because at least three factors can cause chronic instability. First, the patellar groove can be too shallow: if that is the case surgeons can remove some bone to restore it. Second, the tendon that fixesthe kneecap to the tibia might be positioned too far sideways, disturbing kneecap alignment pulling it out of the groove. Surgeons can detach the tendon and reattach it to a better position. Finally, the ligaments that keep the patella inside the groove could be injured and surgeons can reconstruct these.

To help surgeons decide the best combination of surgical interventions, we first developed a system to reliably grade the patellar groove between normal, shallow, flat and convex (Oswestry-Bristol Classification or OBC, Figure 1). Next, we developed a treatment algorithm that used our classification plus a measure of patella alignment to decide the best treatment combination. For instance, only patients with a convex groove (severe abnormality) need their groove deepened, but shallow or flat grooves can be left as they are. We demonstrated that patients treated according to the algorithm had a good clinical outcome with very few complications.

Finally, we studied if patients’ body mass index (BMI) had any effect on the result of treatment. The BMI based on a person’s weight and height and is often used as a rule-of-thumb to classify someone as underweight, normal weight (BMI=18.5- 25), overweight or obese. Patients answered questions about their general quality of life, knee pain and how well their knee functions. Importantly, all patients had a better general quality of life and a better knee after the operation, regardless of BMI. However, we also found that the relations between BMI and general quality of life or knee function had an inverted-J shape (Figure 2). Patients with a BMI around 28 had the best quality of life and those with a BMI around 20-21 had the best knee function after surgery. Studies based on other patient groups also found that people with a BMI around 25, the boundary between normal weight and overweight, report the best general quality of life. However, when it comes to knee function the “best” BMI is in the lower range of normal weight, suggesting that weight loss will benefit all patients but underweight ones