This group of patients, in contrast to knee replacement patients, are usually high-functioning in terms of their knees before the abrupt incident - generally young and fit, with no prior muscle wasting, bony deformity or limitation in their range-of-motion.
After their injury they may be over-keen to get ligament reconstructive surgery done so that they can get back to their sporting activities, and they may have high expectations of a full recovery of function. Yet this is a group that tends to have a high incidence of knee stiffness during their rehabilitation!
When things go wrong, the classical story takes this path - things seem at first to be proceeding well, and rehab accelerates towards the final phase of re-building strength and endurance. Then progress suddenly stops, and the knee becomes red, painful and swollen again. Range-of-motion starts to go backwards, despite 'pushing through the pain'.
A slightly different scenario may be that things seem to be going reasonably well, but the patient finds that they cannot fully straighten because something seems to block those last few degrees of movement.
So what is happening here?
This scenario is likely to be due to inflammation triggering adhesions within the normally mobile spaces above and around the patella, and in the capsular gutters on the sides of the knee. If this progresses, and appropriate and timely intervention does not halt the process, then the adhesions may turn into scar tissue,and earn the name of 'arthrofibrosis'.
Another problem, but usually later in the recovery phase, is the development of a 'cyclops' lesion - a bunching up of fibrous tissue overlying the new graft in the notch area. This can also block the patient from achieving full extension, when they were able to do this earlier in rehabilitation. If the physiotherapists pushes the patient too hard, there may be areas of haemorrhage over the cyclops lesion, and attempts to achieve full extension may be painful.