Controlling sheer stress and load is more complex in the patello femoral joint. The surgeon needs to specify the contact position of the articular cartilage so that exercises can be undertaken outside of this particular movement.
The principle here is that when the area of repair is on the distal part of the patella or proximal trochlea then the point of contact, i.e. loading on the graft with high shear force, is from 0 – 40 degrees of flexion. If the repair is on the proximal part of the patella or distal on the trochlea then the danger flexion zone is 45 degrees onwards.
As shear forces will put a dangerous load on the new tissue then this range needs to be avoided when undertaking strength work. Of course it has to be remembered that in order to position the knee in a safe range then the knee needs to move through the danger range – hence the need to advise patients on how to get on and off a couch or bed etc in order to do their exercises.
Over the first 3 to 5 months, therefore, open kinetic chain exercises which are normally aimed at strengthening the quadriceps should be avoided. The excessive stress during open chain exercises will easily damage repair tissue and it is best to stick with closed chain exercise until at least 3 months and they should not be the predominant method until 6 months.
Knowing the particular point of articulation that involves load on the new grafted area means that open chain exercises can be introduced earlier, provided the movement is in the arc that does not overload the graft. Fast joint movements under load will result in higher load on articular surface and these also need to be avoided initially.
Two other factors are vital to reduce shear forces on the graft and to help control the progression of loading the new surface. These are firstly controlling patello-femoral instability with good distal quads control and secondly control of proximal instability with good hip and core stability. Rehabilitation exercises therefore need to include activities that maintain hip strength and balance.