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Dislocators with normal anatomy prior to dislocation (part 3b of a course on realignment osteotomy)

If you look at the patients with patellar instability, they are a diverse group. As I mentioned in the previous section of this course, the two main things that differentiate the these patients are whether they started off with normal anatomy or abnormal anatomy.

Those patients with normal anatomy are those whose kneecap sits in the trochlea, the tissues are all tensioned appropriately, the patella is the right shape, the trochlea is the right shape and their leg is straight. Then they have an accident where they fall over and they rupture the structures on the inside of the knee. The most important stabilising structure that they rupture on the inside of the knee is the medial (inner) patellofemoral ligament (MPFL), the ligament that goes from the thigh (femur) to the patella. This kind of injury is quite common, and often patients will be mis-diagnosed as having ruptured their ACL. One has to be very careful to ensure that the whole problem is not related to an unstable kneecap with a rupture of the medial retinaculum and MPFL.

MPFL rupture

This kind of injury often occurs on the sports field, when the patient sustains a direct hit to the inner aspect of the knee, or their knee twists awkwardly and the patella is knocked out of its groove. Sometimes the patella returns to the normal position spontaneously or the patient attends Accident and Emergency where the patella is reduced by the doctors. Another common scenario is the patient who injures their knee on a dancfloor after one too many drinks on saturday night out.

This common problem has been mis-managed for years, with the knee put into a plaster cast for a period of four weeks, after which the patient was basically discharged. What we have learned over the last five to ten years is that the last thing you should do after this kind of injury is to immobilise the knee. Of course the patient should rest their knee for a few days – it will be painful and swollen and difficult to use. Ice, compression and anti-inflammatory tablets may further help reduce the swelling. The patient may benefit from the use of a brace (a cricket-pad splint or some form of immobilisation for a few days) but basically they need to start exercising the knee, and in particular the inner quadriceps muscle (the VMO or vastus medialis obliquus). This medial quadriceps muscle is a key player in keeping the kneecap sitting where it is supposed to sit. If you immobilise the knee in a plaster for a long time this muscle will waste away. It is really important that patients start exercising their VMO muscle.

These patients should be seen by an orthopaedic specialist, (preferably a knee specialist) and have appropriate X-rays taken to make sure they haven’t knocked a bit of bone off their patella (osteochondral fracture). If this is the case the patient may need urgent surgical treatment to reattach the broken piece of bone back onto to the patella. First time dislocators with no obvious osteochondral injury do very well with a little bit of rest and then just getting the knee gently moving again with physiotherapy aimed at strengthening the VMO muscle. Surgery is certainly not considered for these patients. If they dislocate their patella again quite soon, and it happens for a very trivial reason then surgery may be more likely. And if it happens three or more times then surgery is what we recommend. Every time you dislocate the patella you run the risk of damaging the underlying cartilage. The cartilage layer can peel off or be knocked off the patella with a block of bone attached to it - this is and osteochondral fracture, as mentioned previously. Leaving the patella unstable can therefore lead to patello-femoral arthritis.

MPFL Reconstruction

MPFL reconstruction

The MPFL reconstruction is a reconstruction of the medial patellofemoral ligament. This ligament is often torn or stretched when patients dislocate their kneecap and its deficiency renders the patella unstable. MPFL reconstruction is presently a very popular talking point amongst surgeons. Various reconstruction techniques are being discussed with enthusiasm, much in the same way as ACL surgery was talked about 10-15 years ago. For MPFL reconstruction, we use one of the smaller hamstrings tendons (gracillis tendon). This tendon is only 3-4 mm in diameter and is borrowed through a small incision just below the knee.

There are many ways of doing this operation, but most surgeons now fix the new ligament either through a little tunnel in the patella by drilling a small hole through the kneecap on the medial side which the tendon is then passed through. The new ligament is then fixed with a little screw to the femur under appropriate tension, thereby re-creating the MPFL ligament. I usually use a metal screw as I found that the ‘dissolving’ screws do not work as well. This procedure works extremely well and stabilises the kneecap immediately. The post-operative rehab is extremely quick. It is a day-case operation - you come in and go home again either the same day or the next day. No knee brace or crutches are needed, although sone patients prefer to use crutches for the first few days. The physiotherapy starts soon and the aim is to regain knee flexion (bendind) as soon as possible. It is just the first 20-30 degrees of flexion that you need for the kneecap to engage in the trochlear groove of the femur – once it is in this groove it stays in the groove. Not that many surgeons are that comfortable doing an MPFL reconstruction because it is quite new, so we get referrals from far afield to do this operation which is actually relatively straight forward.

 

A number of surgeons have performed many MPFL reconstructions and the results of the surgery have been good – this operation is very effective. As I said earlier, we don’t just jump in there with surgery after the first dislocation – we tend to rehab the patients initially with lots of physiotherapy, working on not just the VMO muscle but also the core muscles of the spine, the gluteal muscles and the pelvic floor muscles. It is really important that the patient gets appropriate physiotherapy, and often that will settle things down and the knee will get strong by itself, with most patients settling without the need for surgery. It is only the patients that go on having  recurrent dislocations that we offer the surgical procedure to.

The MPFL reconstruction is arthroscopically-assissted. I think one of the key things is to visualise the kneecap within the knee at the beginning of the operation and then visualise it at the end of the operation through the superolateral portal, which is not the usual viewing portal that we use in arthroscopy. Through this portal we can look from above and see how the kneecap is moving. Once we are happy with its movement and position we then fix it in place with an appropriate amount of tightness. As you are pulling the ligament in position (with the appropriate tension applied) you can see when the patella is in the most favourable position. Then you stop and fix it in that position. You don’t want to overtighten it and pull the kneecap too far medially.

Now this is a course on osteotomy, and MPFL reconstruction is not an osteotomy but a soft tissue procedure, but I have included it here for completeness as the next section will go on to discuss the osteotomy procedures that address patellar dislocation in patients that have abnormal anatomy prior to their first dislocation.

Updated: 11 Apr, 2013
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