This paper distinguishes between -
developmental patella infera - where the patella is moving down into a lower position due to adhesions and scarring, but which situation is still potentially reversible.
- permanent patella infera - where the situation is effectively irreversible, and salvage procedures can only try to improve the mechanics, not restore them.
Using a control group of 51 patients with minor knee problems and no prior surgical procedure on the knee nor any period of immobilisation, the authors established in their population the normal range of vertical height of the patella above the tibial plateau. They also compared the two knees of each patient as well as the reliability of serial measurements in the same patient.
They found, using three different measurement methods, that there was essentially no difference in the patellar height ratios between right and left knees (only 11-15%), but there was a large variation in the ratios from one individual to another.
Having established this baseline, they followed a series of five patients who had developed patella infera during the rehabilitation period or within six months, and whose patellar height ratio was decreased by at least 11-15%. The authors felt that in all cases they were still in the transient ('developmental') stage and that the patella infera was still amenable to surgical restoration.
They drew attention to the need to distinguish this from the syndrome of reflex sympathetic dystrophy (which is a syndrome where excessive pain, out of proportion to the findings, is association with other signs and symptoms due to nervous system changes - funny feelings in the skin, funny coloured blotches etcetera)..
The authors found that in the early development of patella infera the vertical height ratios between the two knees may differ by only a few millimetres. A difference greater than 15% may be considered abnormal. A second X-ray needs to be taken a week or two later to measure any further descent of the patella. The ratio must be compared to the opposite knee or a pre operative X-ray.