The patella, also known as the “knee cap” sits at the front part of the knee. As the knee bends, it glides in a groove of the femur called the “trochlear groove”. This groove accommodates the patella so that it smoothly tracks in a straight line. Other factors that hold the patella in its groove are the pull of the quadriceps muscle and surrounding ligaments. When the patella slips out of this groove, the condition is called “patellofemoral instability” (PFI). [Figure 1]
Figure 1. Patellofemoral instability
In most people with PFI, the trochlear groove and the reciprocal shape of the patella can be abnormally flat. This predisposes patients to having the patella potentially coming out of place. Sometimes the patella sits high relative to the trochlea as well which makes it difficult to enter the trochlear groove. The other reasons include having a weak quadriceps muscle, having lax joints or knocked knees. These anatomic variations may run in families. Some times, there are no underlying factors except a significant traumatic injury to your knee.
Recent research has demonstrated that the main stabiliser of the patella, preventing it from moving outwards (lateral) is a ligament called the medial patellofemoral ligament (MPFL). [Figure 2] This ligament is a thin structure that runs at the bottom part of the VMO muscle (lower inner part of the quadriceps muscle). It starts from the inner (medial) edge of the patella and runs across to the inner part of the femur, 2cm above the knee joint. This ligament is torn or becomes stretched when patients dislocate their patella. It then stops functioning, leading to recurrent instability in some patients.
Figure 2. Knee Anatomy from the frontal aspect and the MPF
You are administered a general anaesthetic (put off to sleep) via medication delivered in to your vein. A medial patellofemoral ligament reconstruction involves making a new ligament to compensate for the damaged one. One of your hamstring tendons (gracilis or semitendinosus tendon) are removed through a small incision at the front inner part of the tibia bone. Approximately a 20cm length of tendon is removed and then fashioned into a new ligament. Several other small incisions are then made at the front of the knee to reconstruct this ligament from the front inner edge of the patella to the inner (medial) edge of the femur. This MPFL is held into position with plastic screws or sutures placed in to small drill holes. [Figure 3]
Figure 3. MPFL graft held in position with plastic screws in to patella and femur
The operation involves approximately three small incisions each measuring between 2cm and 4cm long, but also can be done through one large incision. The number of incisions is dependant on what other surgery is required, such as a tibial tubercle osteotomy (see final part of document). The surgery takes approximately 60-90 minutes to perform.
The surgery is usually performed as a day surgery. When you wake up you will have a bandage on your knee. You will be seen by the physiotherapist and instructed on how to use crutches. You will be allowed to take full weight on your leg. Crutches are normally only required for approximately 3-5 days following the operation, and are used mainly to protect your knee from collapsing whilst the soft tissues are healing.
The main aims of early rehabilitation include reducing swelling, applying compression to the knee, elevating your leg and gently tightening muscles so as to minimise any muscle wasting. You will be asked to see your physiotherapist for regular visits, often 2-3 times a week for the first few weeks and then once a week following this. The main aims of physiotherapy in the early period are to restore some motion of the knee joint, reduce swelling and maintain muscle strength. A specific rehabilitation program will need to be given to your physiotherapist so as they comply with the instructions required for such an operation. This will be given to your by your surgeon following your operation.
A patellofemoral stabilising brace is required after MPFL reconstructive surgery whilst the ligament heals. This is used for the first 6-12 weeks following surgery. This should be fitted and arranged prior to your operation.
It is common to have some pain after surgery but this will be controlled with regular anti-inflammatory medications for the first five days. You will also be given other medications that can be taken as required to reduce pain. As a general rule, pain is not severe immediately following the surgery as local anaesthetics are used at the time of the operation to reduce significant pain during the early postoperative period. These are administered whilst you are having the operation. It is important to ice your knee regularly, 20 minutes every hour during the first few days, as this also reduces inflammation and decreases pain.
It is normal to have swelling following the operation and some bruising around the leg. Swelling and bruising can occasionally be seen around the ankle and this should not cause concern. Your knee will feel stiff and it is important to gently bend your leg following surgery to overcome this sensation. This will also be worked on with your physiotherapist.
As there are several incisions around the knee, it is common to have areas of numbness surrounding those cuts. These patches of numbness generally dissipate with time. In some patients, numbness is also experienced around the outer part of the leg as there is a nerve adjacent to the gracilis tendon which can be stretched during surgery “saphenous nerve”. This will generally diminish with time but a very small percentage of patients have a permanent area of numbness over the outer side of the leg. In our experience, this has never been incapacitating to patients.
You will also notice that the quadriceps muscle at the front of your knee will decrease (atrophy) in size following your operation. This is experienced by all patients and will recover with appropriate exercise and rehabilitation. It can take many months for this muscle to fully recover and its return in size and strength is a major determining factor on when you can return to full activities.
Most patients require approximately 1-2 weeks off work if they work in an office setting and anywhere between three and five weeks if they work in manual labour. This obviously needs to be discussed with your surgeon prior to surgery.
Most people who have this surgery are able to return to normal activities, including sport. The chance of having further dislocations is generally considered to be between 2 and 4%.
In a small group of patients, additional operations are required in conjunction with medial patellofemoral ligament reconstruction to assist in stabilising the patella.
Please see separate section on tibial tubercle osteotomy, which addresses this component of the surgery in more detail.
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