Knee cap (patellofemoral) instability occurs when the knee cap does not slide centrally within the grove of the thigh bone (Trochlea). The knee cap can partially come out of the groove (subluxation) or completely come out of the groove (dislocation).
Our assessment of patients with patellofemoral instability has improved recently as our knowledge of patello-femoral joint anatomy and its stabilizing factors has increased.
Accurate assessment of the underlying abnormalities in the unstable joint enables formulation of appropriate management plans.
Patellar pain is common in both athletic and nonathletic individuals. Among athletes, men tend to present with more patellofemoral injuries, including traumatic dislocations, than women. In the nonathletic population, women present more commonly with patellar disorders. The three most common causes for patellar dislocation remain: traumatic dislocation, trochlear dysplasia (flattening) and maltracking secondary to bone malalignment. It is essential to understand the underlying cause in order to direct appropriate treatment.
Patellofemoral problems are mainly diagnosed by obtaining a thorough history and performing a physical examination. Imaging studies help confirm the diagnosis. Plain radiography is not as sensitive as magnetic resonance imaging (MRI), but it is the least expensive and most readily available modality.
In general, surgery is more effective in preventing recurrences of dislocation because skeletal and muscular components of the patellofemoral joint and extensor mechanism are realigned; however, surgery also has risks. In a patient with normal anatomy, surgery should be considered an option after all conservative treatment modalities are unsuccessful. Patients with anatomic abnormalities may benefit from earlier surgical consideration.
The most common facts about recurrent instability include:
- Female teenagers are the most common first time dislocators
- After a first time dislocation approximately 17% of patients will experience further problems
- If previous subluxation or dislocation, 50% will have further episodes of instability
- The younger the age at first dislocation, the higher the risk of subsequent dislocation
- More severe initial dislocation = higher risk of recurrent dislocation
- Further risk factors are a family history of patella instability and the risk factors for developmental dysplasia of the hip (first born girl, high birth weight, breech delivery, Caesarian)
- Imaging of the Knee with Chronic Instability
These are the imaging techniques and measurements I use for reliability and simplicity and the benefit they provide in decision making.
- X-rays These can show, Patella alta, Trochlea dysplasia
- CT scans – CT scans have been widely used in the assessment of chronic instability. I mainly use CT scan to assess complex rotational alignment deformity
- Magnetic Resonance Imaging (MRI) – MRI scans will reliably demonstrate
- Bone abnormalities – Trochlea and patella dysplasia
- Large cartilage injuries to the patellofemoral region
- Patella tilt any abnormalities to the static constraints e.g. medial
- patellofemoral ligament rupture and patella alta
- Bone bruising of a recent dislocation will be seen on these images
Although far more expensive, MRI is more effective than CT scanning in determining if patellar chondral lesions are present and for determining cartilage thickness and volume. Assessment of patellar cartilage thickness and volume is important to evaluate for osteoarthritis.
Axial views on MRI or CT scans may be used to evaluate the trochlea-tubercle (TTTG) distance. This is the horizontal distance in a vertical plane between the intercondylar notch and the tibial tubercle. Similar to the Q-angle, this evaluates the potential laterally directed vectors on the patella. Some authors suggest that a distance of 2 cm is specific, but not sensitive, for maltracking.
During the acute phase of a patellar injury or dislocation, the immediate goals are to reduce inflammation, relieve pain, and stop activities that place excessive loads on the patellofemoral joint. Physiotherapy is an essential component of treatment.
Quadriceps strengthening is initiated during the acute phase. In the event of acute patella dislocation, these should be static exercises initiated during the period of immobilization. Reduction of swelling and regaining knee range of motion are also primary goals. Therapy should also include a protocol for hamstring muscle stretching. Tight hamstring muscles functionally counteract their agonist group, the quadriceps.
Patellar taping (McConnell method)
The goals of McConnell taping are to restore proper alignment and control pain
With proper alignment, VMO (quadriceps muscle) retraining is initiated. Once taped, patients should note decreased pain when performing painful activities. The goal of taping is to optimise patellar positioning and facilitate better activation of the medial (inner) patellar stabilisers, particularly the VMO. The technique can be taught to patients to perform themselves. This method was described by a Sydney based physiotherapist and is now used in practice worldwide.
Formulating a Surgical Management Plan
Surgery is only considered if non-operative treatment has failed and the recurrent nature of the disease has resulted in functional impairment.
Surgery may be directed toward either bone or soft tissue components.
I believe the surgical strategy should, wherever possible, aim to restore normal anatomy to the joint, rather than introduce new abnormalities. I will discuss what I believe will be the best option for you.
- Rotational Oteotomy – Breaking the bone and fixing it in a new position to get the knee cap to track better
- Tibial Tubercle Osteotomy – Moving the bony attachment of the patella
Soft Tissue Procedures
- Medial patellofemoral ligament reconstruction – Using the hamstrings tendon to reconstruct the MPFL
- Lateral release – Releasing tight tissues on the outside of the patella
- Medial imbrication – Moving the muscle acting on the patella
Presence of Arthritic Change in the Patellofemoral Joint
In the presence of degenerative joint disease (cartilage damage), surgical treatment becomes difficult with a guarded outcome. If the articular lesion is localized to only one side of the joint then cartilage healing (micro fracture) or cartilage restorative (eg MACI or OATS) procedures can be used.
If degeneration is present on both joint surfaces, salvage surgery will be required.
Options include chondroplasty(shaving loose cartilage), lateral release (releasing tight tissue on the side of the knee cap) and tibial tubercle elevation (reseting the position of where the patella tendon attaches).
With failed surgery or advanced arthritis (wear and tear), partial knee replacement or total knee replacement are the only options.