Patella Dislocation and Pain (the kneecap)
The patella, more commonly known as the kneecap, is a small bone sitting over the front of the knee, the femur is the long bone of the thigh. The patella bone lies within the tendon of the quadriceps muscle and moves up and down on the femur bone behind it – this region is called the ‘Patellofemoral Joint’. The kneecap is held in place by several ligaments on either side and by the patellar tendon at the farthest part. The quadriceps muscle is the large, four-part muscle along the front of the thigh. This compartment can be compared to a rope running over a pulley, with the rope being the patella and thigh muscle (quadriceps), and the pulley being the front facing side of the femur (Trochlear groove).
The back of the patella is covered with smooth, shiny white joint cartilage and the undersurface of the patella is v-shaped and sits within a groove in the underlying femur known as the ‘trochlear groove’. Although the patella is free to slide up and down the groove as the leg bends and straightens, it is not free to move from side to side as it is contained by the walls of this groove. This joint is very complex and it has been calculated that the loads here can be up to 12 times body weight!
Patellar instability presents either as a subtle abnormal tracking of the knee cap which can be occasionally painful or as an acute patellar dislocation that requires emergency admission to hospital.
Patellar dislocation is a common problem in the younger and athletic population and can be more disabling than cruciate ligament injuries.
After a first time patellar dislocation there is a 17-49% risk of re-dislocation. It is particularly higher in patients below 20 years of age. The risk increases to 44-71% following a second dislocation. Therefore, most surgeons would recommend surgical intervention after the second time dislocation. Recurrent patella dislocation may eventually lead to progressive cartilage damage and severe osteoarthritis (OA) if not treated adequately. The risk of OA is 35% if the patient ignores the problem.
The medial patellofemoral ligament reconstruction (MPFL)
VMO and MPFL after removing the skin in a the Lab
This is a 5 cm long ligament in the inside of the knee attached to the upper half of the patella and an area between the adductor tubercle and the medial epicondyle of the femur. The ligament acts as a check rein to stop the patella from moving out. This ligament gets injured in 95% of the patients who dislocate their patella. Therefore, the ligament often needs to be reconstructed in order to regain stability in the knee cap. Without the medial patellofemoral ligament, the kneecap dislocates laterally (outer side of the knee). Because the medial patellofemoral ligament is connected with other ligamentous structures, complete rupture will likely damage other areas as well.
Contributing factors to patella dislocation (Why the patella dislocates)
The risk of patellar dislocation can be increased by abnormal anatomical factors. These include generalised hypermobility (24%), knee cap hypermobility (51%), increased thigh bone rotation (27%), pelvic core and hip muscle weakness, abnormal knee rotation, shallow trochlea, knocked knees, high pattella, muscle and soft tissue imbalance, external tibial torsion and foot hyperpronation.
Functional rehabilitation is the mainstay of non-operative management with particular focus on normal walking, core stability and quadriceps strengthening. Currently, we treat acute first time dislocation by relocating the patella back on top of the knee. Non-operative treatment is indicated in acute first-time dislocation without associated osteochondral (bone and cartilage) fracture or loose bodies. Patients will generally be provided with advice to elevate the leg, apply ice and wear a splint for a short period of time. They may then be referred for a course of physiotherapy to rehabilitate the knee and strengthen the muscles around the joint. Despite the high rate of re-dislocation after first time patella dislocation, the benefit of acute immediate soft tissue repair or reconstruction is yet to be established.
The principles of surgical management in patients with recurrent instability is to address the primary abnormal anatomical factor that contributes most to re-dislocation without creating a secondary pathoanatomy to compensate for it.
In summary most of the time the patella can be stabilised by a soft tissue operation called medial patellofemoral ligament reconstruction (MPFL) which involves harvesting a small tendon from around the knee and fixing it to the knee cap and the thigh bone to replace the damaged ligament.
The surgical options are as demonstrated in the table below:-
The principles of surgical intervention
|Instability with Malalignment
||Tibial Tuberosity Medialisation
|Instability without Malalignment
|Instability with patella alta
||Tibial Tuberosity Distalisation
Unfortunately, it is never as straightforward as the summary suggests. Often there are multiple abnormal anatomical factors that are interacting in the background. An event that leads to first-time dislocation disrupts knee homeostasis (harmony) and causes it to decompensate. Homeostasis can be restored by simpler procedures such as medial patellofemoral ligament reconstruction (MPFL) in more than 80% of the cases. However, in certain patients the patella is permanently dislocated or tracking in the lateral gutter (on the side of the knee), only relocating (back on the top) in full knee extension. This group of patients would require more than one procedure to achieve patellar stability. The success of surgery is very much dependent on accurate clinical assessment, appropriate choice of surgical intervention and finally and most importantly on the surgical technique.