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Patellofemoral Joint

What is the Patellofemoral Joint

The Patellofemoral Joint (PFJ) is the articulation between the patella (kneecap) and the trochlea (the groove on the end of the thigh bone that the patella glides on). Problems with the PFJ relate to poor mechanics between the patella and the trochlea. This can present as pain, poor tracking of the patella over the trochlea or dislocation of the patella. Over time this frequently leads to arthritis.

Anatomy of the PFJ

The tongue in groove articulation between the patella and the femoral trochlea is affected by a number of factors, any combination of which can lead to problems with the way the patella relates to the femoral trochlea. These factors include:

  • Trochlear Dysplasia. Trochlear Dysplasia refers to femoral trochlear anatomy. In a normal situation the trochlea forms a deep groove of constant depth. Once the patella is engaged in this groove it is extremely difficult to dislocate. In Trochlear Dysplasia the groove is too shallow in which case the trochlea does not adequately contain the patella until the knee is in deep flexion. This leaves the patella vulnerable to dislocation early in the knee flexion arc which is the position in which the foot strikes the ground when running or landing from a jump.

  • Patella Alta. Patella Alta means that the patella sits too high in relation to the femoral trochlea. As a result of this the patella does not properly engage the trochlea groove until the knee is in deeper flexion. As with Trochlear Dysplasia, Patella Alta leaves the patella vulnerable to dislocation early in the flexion arc. A combination of Patella Alta and Trochlear Dysplasia is an extremely potent one and is associated with a high risk of recurrent patellar dislocation.

  • Medial Patellofemoral Ligament (MPFL) Insufficiency. The MPFL stops the patella dislocating in the first 20º of the flexion arc and is therefore particularly important in individuals with Patella Alta and/or Trochlear Dysplasia. A traumatic patellofemoral dislocation usually results in a rupture of the MPFL. This generally does not heal well and results in an increased risk of further dislocations, particularly in those with Patella Alta and/or Trochlear Dysplasia.

  • Increased Quadriceps Angle. Quadriceps angle refers to the angle of divergence between the line of pull of the quadriceps and the line of pull of the patella ligament. The wider this angle the harder the quadriceps muscle will try to dislocate the patella when it contracts.

  • Ligamentous Hyperlaxity: Hyperlaxity (loose ligaments) is not necessarily an abnormality by itself but a reflection of the natural variation between individuals. In the context of Trochlear Dysplasia and Patella Alta a lax MPFL further increases the risk of dislocation.​

Other factors do affect patellofemoral instability but the five factors outlined above are critical with respect to treatment and surgical decision making.

Individualised Surgery for the PFJ

Many cases of PFJ problems will not require surgery. Strengthening, stretching, biomechanical retraining and activity modification will get many over the line. Patellofemoral dislocations can do a lot of damage to the patella so predicting those at risk of ongoing disloactions is important. Age and Trochlear Dysplasia are the most predictive factors. If you are younger than 25 and have Trochlear Dysplasia you at pretty high risk of dislocating again.

In the past, all individuals with patellofemoral instability underwent similar surgery regardless of what anatomic problems were causing the patella to dislocate. The state of the art approach to patellofemoral instability is to precisely measure the affect of the four factors outlined above and to correct them as required. These factors are assessed with a clinical examination and with an xray, CT scan and MRI scan. Surgical treatment can then be individualized to correct any one individual's specific anatomic deficiencies.

The aim is to correct biomechanics as well as prevent dislocations while doing as little as possible in terms of surgical intrusion.

Tibial Tubercle Osteotomy

A tibial tuberosity osteotomy (TTO) is done to treat Patella Alta and/or an increased quadriceps angle.  The patella ligament runs from the patella to the top of the tibia.  Patella Alta is treated by moving the bony attachment of the patella ligament on the tibia distally (downwards) and an increased quadriceps angle is treated by moving the bony attachment of the patella ligament medially (inwards).  A combination of Patella Alta with an increased Q-angle is treated by moving the bony attachment both downwards and medially. If patellar height and quads angle are normal no osteotomy is done. A TTO is biologically the same as fracture and is treated the same way (rest, ice, elevation, splintage, restriction of weight bearing)

Medial Patellofemoral Ligament (MPFL) Reconstruction 

A MPFL reconstruction is done by harvesting the small Gracilis tendon from the back of the leg through a 3cm incision on the shin.  The Gracilis tendon is doubled and fixed to bone tunnels in the patella and the femur through small incisions. The graft is tensioned so that the patella can no longer dislocate but is not pulled too hard against the femur. MPFL reconstruction is usually combined with an osteotomy.


A trochleoplasty is a  procedure that involves deepening a severely dysplastic trochlea.   In many cases of mild Trochlear Dysplasia will be associated with either Patella Alta, an increased quadriceps angle, a Medial Patellofemoral Ligament insufficiency or a combination of these conditions and the treatment of these other three conditions is all that is required to control the patella. If the trochlea is severely dysplastic and / or isolated then a Trochleaplasty will be required to control the patella without overloading the articular cartilage.

This link will take you to a video that demonstrates the procedure but be warned that the video is quite graphic and could be upsetting.

Corrected mechanics

The aim of PF surgery is not to only control poor tracking / dislocation but to optimise mechanics so that the likelihood of patellofemoral pain and arthritis is minimised. Indvidualised surgery that corrects pathology and leaves normal alone is the key to achieving these goals. As seen in the dynamic CT scans below patellofemoral mechanics  can be dramatically improved.

Surgical journey

Most PFJ surgery requires one or two nights in hospital. A combination anaesthetic is used to optimise pain control. Getting around involves a splint and crutches and begins on the day of surgery. Structured physiotherapy begins on the day after surgery. Elevation, ice, rest, DVT prevention and gentle physiotherapy make up the first 2 weeks. Restrictions on weight bearing and range of motion vary from 0 days to 6 weeks depending on the combination of surgery used. A structured recovery guided by a physiotherapist takes around 6 months from start to finish.

Risks and Complications

As with any major surgery, there are potential risks involved. The decision to proceed with the surgery is made because the advantage of surgery outweighs the potential disadvantages. It is important that you are informed of these risks before the surgery takes place. Complications can be general medical complications or local complications specific to the knee. Patellofemoral realignment surgery is usually undertaken on young healthy individuals and general medical complications are uncommon but possible. Complications include:

  • Allergic reactions to medications

  • Side effects to painkilling medications such as nausea and vomiting

Local complications:

  • Infection. Infection can occur with any operation in the knee.  This can be superficial or deep.  Infection can have very serious consequences and may require a re-admission to hospital and further surgery.

  • Blood Clots (Deep Venous Thrombosis). These can form in the calf muscles and travel to the lung (a pulmonary embolism).  These can occasionally be serious and even life threatening.  Any calf pain or shortness of breath in the early postoperative period should be notified immediately.  The key to avoiding DVTs is mobility.  It is not so much the time spent mobilizing but the frequency of mobilization that is important.  In the early postoperative period a lot of small movements is the best prevention.  Other prevention strategies will be employed and individualized to the patient.

  • Stiffness. The aim of patellofemoral realignment surgery is to produce a stable knee without restriction of movement.  Loss of movement following patellofemoral realignment surgery is very uncommon but control of swelling and attention to home exercise program and physiotherapy appointments is an important part of regaining full movement as early as possible.

  • Nerve Injury. Patellofemoral realignment surgery is not particularly threatening surgery to any major nerves.  A small nerve that crosses just below the insertion of the patellar tendon on the tibia is routinely cut and results in numbness in the upper outside part of the calf.  Most people are very aware of this numbness early on but the sensation tends to fade with time.

  • Recurrent Instability. Further dislocations following instability are a rare event but are possible if enough force is applied to the joint.

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