Structured oral examination question 8: Patellar instability
EXAMINER: A 17-year-old lady is referred to your Patella clinic by the GP due to recurrent bilateral patellar dislocation. How would you assess this patient?
CANDIDATE: I would start by taking a detailed history followed by clinical examination. In the history, I would enquire about age at first dislocation, frequency of dislocations, traumatic or atraumatic, any associated syndromes such as bone or connective tissue dysplasia and generalized joint laxity. I would also enquire about any mechanical symptoms, the presence and localization of pain.
EXAMINER: What are risk factors for patellar instability?
CANDIDATE: The risk factors for patellar instability are:
- Bony factors (static)
Trochlear dysplasia.
Hypoplastic femoral condyle.
Patellar shape. Patella alta.
- Malalignment
Patellar malalignment is an abnormal rotational or translational deviation of the patella along any axis.
External tibial torsion/foot pronation.
Increased femoral anteversion and increased genu valgum.
Increased Q angle or abnormal tibial tuberosity–trochlear groove (TT–TG) distance.
- Soft tissue (dynamic)
Ligamentous laxity (medial patellofemoral ligament rupture/ insufficiency).
- Abnormal gait
Walking with valgus thrust.
- Genetic factors such as connective tissue disorder syndromes.
EXAMINER: Tell me about the most important static stabilizer of the patella.
CANDIDATE: The primary static restraint to the lateral patellar displacement is medial patellofemoral ligament. It provides 50% of the total medial restraining force. MPFL sectioning can lead to substantial changes in patellar tracking. It originates from the area between the medial epicondyle and adductor tubercle and inserts onto the proximal two-thirds of the patella. The average length of the ligament is 5.5 cm. During acute patellar dislocation there is a 90–95% incidence of damage to the MPFL. Femoral attachment is commonly affected. In the past 10 years, MPFL reconstruction has become a popular procedure for treatment of recurrent patellar dislocation.
EXAMINER: How would you investigate this patient?
CANDIDATE: I would perform the following investigations:
- A lateral radiograph is the most helpful view for assessment of patellar tilt, height and trochlear depth.
- Axial radiographs (Merchant’s view) to assess patellar tilt angle (normal < 10), congruence, sulcus angle ( normal 138) and trochlear dysplasia.
- MRI for articular lesion and state of MPFL.
- CT scan to assess:
Femoral anteversion (normal 5–15) Tibial torsion.
TT–TG distance more than 15–20 mm is significant.
Patellar tilt.
Trochlear depth.
EXAMINER: What are the principles and methods of distal realignment procedures?
CANDIDATE: The three main groups of realignment procedure as determined by direction of tibial tubercle (TT) transfer are:
Medial transfer to treat malalignment.
Anteromedial transfer for malalignment and PFJ chondrosis. Anterior when there is distal PFJ chondrosis.
The methods of realignment are:
Elmslie–Trillat: Medialization without posteriorization of the tibial tubercle.
Fulkerson: Medialization with anteriorization of the tibial tubercle in the arthritic patella.The obliquity of the cut depends on the degree of malalignment and arthrosis. A steep cut up to a 60 angle maximizes anteriorization and is useful in patients who have more arthrosis than malalignment.
Hauser: Transfer of the tibial tubercle to a medial, distal and posterior position. This has been abandoned. It increases the PFJ reaction force and causes patellofemoral degenerative joint disease.
Goldthwait 1899–Roux 1888: Medial transposition of the medial half of the patellar tendon, lateral release/medial reefing. Now the lateral half is placed under the medial half and medially (historical procedure) .
Maquet: Anterior transportation of tibial tubercle, which decreases patellofemoral contact forces. Not performed nowadays (historical) as it has a high incide
