If problems like mal-alignment are not managed well they may progress to the point where the cartilage on the undersurface of the patella becomes damaged and arthritis results.
Healing of joint cartilage is a real problem. Joint cartilage is a complex structure - the cartilage cells lie, each in its own small cavity, singly or in little clumps, within a special matrix containing fibrils of collagen. This matrix gives the cartilage its special resistance to wear and tear.
There are no nerves or blood vessels. The cartilage is nourished by diffusion from the underlying bone and by being washed on its upper surface by the synovial fluid.
When cartilage is damaged, it heals poorly, and any new cartilage tends to be of a poorer quality - so-called 'fibrocartilage'. These fibrocartilage cells form from undifferentiated 'stem' cells in the underlying bone marrow.
A number of surgical options to slow down early destruction of the joint surface of the patello-femoral joint have become standard -
- abrasion arthroplasty
- OATS (osteoarticular transfer system)
- ACI (autologous chondrocyte implantation)
I'll take you briefly through each of these. They all fall under the heading of 'cartilage restoration' techniques. This is a big subject, but I am just going to give you an overview. The topic will be covered in greater depth in other KNEEguru courses and papers.
A rotating burr is used to rub the damaged joint cartilage away, revealing bleeding bone underneath, from which the cells are stimulated into producing fibrocartilage.
Unfortunately the burr can cause heat damage to the cells.
Microfracture avoids the heat damage. A 'pick' is used to poke a hole into the underlying bone, again to stimulate the deeper cells into producing fibrocartilage.
The poor mechanical property of fibrocartilage encouraged surgeons to develop techniques where proper cartilage filled the damaged area, not fibrocartilage.
In the OATS technique small plugs of healthy cartilage, together with a bit of underlying bone, are taken from the edges of the trochlea and placed in holes in the damaged surface.
Where several small plugs are packed together like a mosaic, the procedure is called 'mosaicplasty'.
ACI takes the concept even further. A minor operation is done to take some healthy cartilage cells The cells are sent away to a laboratory and cultured to increase the number of cells In a second surgical procedure these cultivated cells are pushed into the defect and held in place by a piece of fibrous tissue which is sewn over them like a taut tent.
This is a very expensive two-operation procedure.
These procedures require that the patient be non-weight-bearing on crutches for several weeks, and failure of the procedure is frequently blamed on poor compliance.
Severe arthritis limited to just the patellofemoral joint is rare, and, accordingly, the indications for a patellofemoral replacement are limited.
Unfortunately, when the patellofemoral joint is arthritic there are few reliable surgical options. All of the operations listed above may move and unload the kneecap to a certain extent, and restore the joint surface to some extent, but none will predictably relieve pain. Removing worn out bone and 're-surfacing' the remainder of the bone with metal and/or plastic becomes the most reliable option.
The traditional way to this is to insert a 'total knee replacement' whereby all three compartments of the knee are resurfaced. A more conservative approach is to simply resurface the trochlear groove and the patella, the so-called patellofemoral replacement - but note that most orthopaedic surgeons have never performed such a procedure.
[If you're a purist the PF replacement in our illustration doesn't look very good. The trochlea is pointing laterally, and, as such, there isn't a good geometric fit between the patella and trochlea. It needs a better illustration, but this will need to do for now.]
And, finally, 'patellectomy'.
Removing the kneecap altogether is called a patellectomy. This has largely fallen out of favor, for the knee cap serves a purpose, and once it's out, for all intents and purposes there's no going back.
Remember that the patella acts as a fulcrum for the quadriceps muscles, thus making the knee stronger. Lack of the patella weakens the extensor mechanism.
Patellectomies are now by and large reserved for kneecaps that are infected, shattered or severely thinned out.
If you came out of this lesson with just these two messages, I will be happy -
- Patellar surgery is a specialist domain
- Lateral release should only be performed if the lateral retinacular structures are demonstrably tight
This issue of complications brings up the associated topic of 'informed consent'. Patients want and need to be informed about possible complications.
But a true list of what could go wrong would have a serious effect on the patient's confidence. It is a matter of judgement and experience what the doctor explains to the patient prior to surgery. One thing is certain though - most of the complications occur in the hands of less experienced surgeons.