Patellofemoral instability: understanding kneecap dislocation

What is patellofemoral instability?

The patella — commonly called the kneecap — sits in a groove at the front of the femur called the trochlea. As the knee bends and straightens, the patella tracks up and down within this groove. In a stable knee, a combination of bony anatomy and soft tissue restraints keeps the patella correctly positioned throughout that movement.

Patellofemoral instability occurs when the patella moves excessively or fully dislocates out of the groove — most commonly towards the outer side of the knee. This can happen as a single traumatic event, or as recurrent episodes in patients whose anatomy predisposes them to it. In either case, the experience is alarming: the knee gives way, often accompanied by pain, immediate swelling, and an inability to bear weight.

Who is affected?

Patellofemoral instability most commonly affects young, active people — teenagers and young adults in their twenties and thirties are disproportionately represented. It is somewhat more common in females, partly due to anatomical differences in pelvic width and knee alignment. Certain anatomical features increase the risk substantially:

A shallow or dysplastic trochlear groove provides less mechanical containment for the kneecap. A high-riding patella (patella alta) means the kneecap sits above the groove during early knee flexion, when bony support is greatest. An increased distance between the tibial tubercle and the trochlear groove (TT-TG distance) shifts the pull of the quadriceps muscle in a direction that favours lateral displacement. These factors can be present individually or in combination, and their presence influences both the risk of recurrence and the choice of treatment.

What happens at the time of dislocation?

When the patella dislocates laterally, the medial patellofemoral ligament (MPFL) — the primary soft tissue restraint on the inner side of the kneecap — is torn. This structure runs from the medial border of the patella to the inner side of the femur just above the knee joint. Its disruption is almost universal with a first-time dislocation.

At the moment the patella snaps back into the groove, either spontaneously or when the knee is extended, the undersurface of the patella and the lateral wall of the femoral condyle can sustain cartilage damage. This so-called osteochondral injury may leave loose fragments within the joint and contributes to the pain and swelling seen after dislocation.

Diagnosis and investigation

A careful history and physical examination are central to assessment. Examination of the knee includes assessment of patellar tracking, apprehension testing — where the patient becomes anxious when the patella is pushed laterally — and evaluation of ligament laxity more broadly.

Imaging includes plain X-rays to look for bony injury and patellar height, and an MRI to assess the MPFL, cartilage surfaces, and trochlear morphology. CT scanning is sometimes used to measure the TT-TG distance and to characterise the degree of trochlear dysplasia in detail.

Non-surgical management

For a first-time dislocation in a patient without significant anatomical risk factors, non-surgical management is a reasonable initial approach. The initial period after dislocation involves rest, ice, compression, and elevation to manage swelling. A brace or splint is often used for comfort in the first few weeks.

Physiotherapy is the cornerstone of non-surgical management. Strengthening the muscles around the knee — particularly the VMO (vastus medialis oblique) portion of the quadriceps — improves dynamic patellar stability. Hip abductor and external rotator strengthening is also important, as weakness in these muscle groups affects lower limb alignment and loading patterns.

The limitation of non-surgical management is recurrence. After a first dislocation, studies suggest that roughly 15 to 30 per cent of patients will have a further episode. In patients with significant anatomical risk factors — trochlear dysplasia, patella alta, or increased TT-TG — the recurrence rate is substantially higher. Recurrent instability causes progressive cartilage damage and is a strong indication for surgery.

Surgical options: MPFL reconstruction

Medial patellofemoral ligament (MPFL) reconstruction is the central surgical procedure for recurrent patellofemoral instability in patients with normal or near-normal anatomy. The torn or attenuated MPFL is replaced with a graft — typically a hamstring tendon harvested from the same knee — which is anchored to the medial border of the patella and to the anatomical MPFL footprint on the inner femur.

The goal is to restore the medial restraint that was lost at the time of the original dislocation, preventing the patella from displacing laterally during activity. When performed with careful attention to graft tension and the correct anatomical femoral attachment point, MPFL reconstruction reliably restores stability and allows return to sport and high-level activity.

Addressing anatomical risk factors

In patients with significant trochlear dysplasia or a substantially elevated TT-TG distance, MPFL reconstruction alone may not be sufficient to prevent recurrence. Additional procedures may be considered:

A tibial tubercle osteotomy (TTO) repositions the bony attachment of the patellar tendon on the shin, reducing the lateral pull on the patella and improving its position within the groove. It is typically indicated when the TT-TG distance is markedly elevated, and can be combined with MPFL reconstruction in the same operation.

Trochleoplasty — deepening a shallow groove surgically — is reserved for severe trochlear dysplasia and is performed by a small number of surgeons with specific training in the technique. The decision about which combination of procedures is appropriate requires careful preoperative planning and is tailored to each patient’s anatomy.

Recovery after MPFL reconstruction

Recovery after MPFL reconstruction involves a graduated return to activity over several months. Patients typically use crutches for the first few weeks while regaining strength and confidence. Formal physiotherapy begins early, focusing on range of motion, quadriceps activation, and progressive strengthening.

Return to full sport — including cutting and pivoting activities — generally takes five to seven months. The timeline is longer when additional bony procedures have been performed. Compliance with rehabilitation is important to the outcome: patients who engage consistently with physiotherapy tend to recover more completely.

When to seek advice

If you have experienced a kneecap dislocation — whether a single event or a pattern of recurrent instability — it is worth seeing an orthopaedic surgeon for a thorough assessment. The anatomical factors that determine risk and guide treatment decisions require proper imaging and examination to evaluate. Early intervention, particularly after a first dislocation in a high-risk patient, may prevent the progressive cartilage damage that comes with repeated episodes.

Dr Mooney consults at Orthopaedics 360, 204 Greenhill Road, Eastwood. Contact the rooms on (08) 7077 0157, or arrange a GP referral for Medicare rebate purposes.