Multiple factors affecting static or dynamic lower extremity stabilizers can lead to patellar dislocation, including patella alta, trochlear dysplasia, excessive lateral patellar tilt, increased Q angle, dysplastic vastus medialis obliquus, or overpull from lateral soft tissue structures.1–4 Patellar dislocation often occurs when the knee is extended and the foot is externally rotated.1 Nikku et al5 discovered that injuries commonly occurred as an extended knee was moving into flexion. This injury is commonly the result of noncontact twisting.1,5
Isolated patellar dislocations and femoral nerve lesions have both been well documented. Concomitant femoral nerve lesions with patellar dislocations are less common and not well described. A case of femoral neuropraxia in a dancer, which occurred after multiple quadriceps stretches during routine maneuvers, has been previously described.6 However, the authors are unaware of any case studies presenting femoral nerve lesion with concomitant first-time patellar dislocation.
A 16-year-old female dancer with no prior history of patella instability or lower extremity injury presented following a first-time patellar dislocation sustained during a routine dance maneuver. Her initial physical examination indicated a moderate knee joint effusion with an apparent decrease in the circumference of the left thigh compared with the contra-lateral thigh. She had tenderness along the medial aspect of the patella and lateral femoral condyle.
Magnetic resonance imaging of the left knee (Figure 1) showed a large effusion with partial medial patellofemoral ligament disruption and edema pattern consistent with recent lateral patellar dislocation. She was placed in formal physical therapy and prescribed a lateral patella brace. One month following her initial presentation, she reported improved strength and range of motion but continued difficulty with weakness in knee extension. She also noted numbness and tingling through the anterior left thigh. In addition, she continued to exhibit more pronounced atrophy of the left thigh.
T2-weighted axial magnetic resonance imaging. A nondisplaced cortical disruption at the medial patella margin with adjacent edema and partial disruption of the medial patellofemoral ligament (A). The patellofemoral joint 5 days after injury showing significant joint effusion and bony edema of the lateral femoral condyle and the medial aspect of the patella (B).
Electrodiagnostic studies of the left lower extremity performed 3.5 months after injury indicated axonal femoral neuropathy “at or around the level of the inguinal ligament.” Magnetic resonance imaging of the left thigh at that time also exhibited denervation atrophy noted through the anterior thigh musculature (Figure 2). There were no obvious space-occupying masses or lesions noted through the pelvis. Diagnosis at that time was determined to be a patellar dislocation with concomitant femoral nerve palsy.
T1-weighted axial magnetic resonance imaging of the right (A) and left (B) thighs 3.5 months after injury showing left anterior thigh muscular denervation atrophy.
Five months after her injury, the patient exhibited improved hip flexion and knee extension strength, with decreased thigh paresthesias. She reported continued weakness through the quadriceps musculature but demonstrated improvement in knee extension strength to 4+/5 on manual muscle testing. A 4-cm difference in circumference of the left thigh compared with the right persisted.
Thirteen months following injury, improvements in quadriceps strength and stability were again noted, but continued subjective knee extension weakness was reported and circumferential thigh discrepancy was still visually apparent.
Electrodiagnostic studies were repeated 2 years following her injury, which showed normalization of the left femoral nerve function. At her most recent follow-up—2.5 years after injury—the patient continued to report slow improvements in her left thigh strength and knee stability, with full (5/5) strength noted in knee extension on manual muscle testing. However, she continued to report subjective weakness in knee extension and had not been able to return to competitive dance.
The knee is a complex joint commonly under increased strain during sporting activities, with 20% to 25% of all knee injuries occurring during these activities.4 The patellofemoral joint receives static stability from osseous and soft tissue constraints. It also receives dynamic stability primarily from the vastus medialis obliquus. Osseous stability is provided by the depth of the trochlear groove together with the height of the lateral trochlea. Static stability is provided by the medial patellofemoral ligament, medial patellomeniscal ligament, medial patellotibial ligament, medial retinaculum, and joint capsule.2 Patellar dislocation is a common injury, accounting for 3.3% of all knee injuries according to Majewski et al.4 A Danish study performed by Gravesen et al3 found that female patients 10 to 17 years old seem to have the highest incidence of dislocation, at a rate of 0.108% (108 per 100,000).
Isolated femoral nerve lesions typically occur by way of stretch, compression, or direct trauma.7 Miller and Benedict8 reported the case of a ballet dancer who sustained bilateral femoral nerve lesions from performing a maneuver that placed her knee in hyperflexion and her hip in extension, thereby placing the femoral nerve on stretch. Another case of femoral neuropraxia in a dancer was described by Sammarco and Stephens.6 They concluded that this dancer's lesion occurred after multiple mild quadriceps stretches during routine dance maneuvers.
The current authors suggest that the femoral nerve palsy in their case possibly preceded the patellar dislocation, as evidenced by the thigh circumference difference already noted at the time of primary presentation. This femoral nerve stretch injury may have been created by recurrent excessive stretches performed through the quadriceps musculature, secondary to numerous dance practices and competitions. This in turn may have contributed to the quadriceps weakness that allowed for the lateral patellar dislocation to occur during a routine and otherwise atraumatic dance maneuver. Other case reports have suggested that a dislocation of the patella caused a pull on the femoral nerve, resulting in femoral nerve palsy.7
Although patellar dislocations and isolated femoral nerve palsies occur with relative frequency, it is uncommon for them to occur concomitantly. Further studies evaluating these 2 injuries may shed light on their true cause–effect relationship and could help to develop training protocols to reduce their incidence.
- Abbasi D, McCulloch P. Patellar instability. http://www.orthobullets.com/sports/3020/patellar-instability?expandLeftMenu=true. Accessed July 29, 2017.
- Duerr RA, Chauhan A, Frank DA, Demeo PJ, Akhavan S. An algorithm for diagnosing and treating primary and recurrent patellar instability. JBJS Rev. 2016;4(9):1–11. doi:10.2106/JBJS.RVW.15.00102 [CrossRef]
- Gravesen KS, Kallemose T, Blønd L, Troelsen A, Barfod KW. High incidence of acute and recurrent patellar dislocations: a retrospective nationwide epidemiological study involving 24,154 primary dislocations. Knee Surg Sports Traumatol Arthrosc. 2018;26(4):1–6.
- Majewski M, Susanne H, Klaus S. Epidemiology of athletic knee injuries: a 10-year study. Knee. 2006;13(3):184–188. doi:10.1016/j.knee.2006.01.005 [CrossRef]
- Nikku R, Nietosvaara Y, Aalto K, Kallio PE. The mechanism of primary patellar dislocation. Acta Orthop. 2009;80(4):432–434. doi:10.3109/17453670903110634 [CrossRef]
- Sammarco GJ, Stephens MM. Neurapraxia of the femoral nerve in a modern dancer. Am J Sports Med. 1991;19(4):413–414. doi:10.1177/036354659101900417 [CrossRef]
- Shin CS, Davis BA. Femoral neuropathy due to patellar dislocation in a theatrical and jazz dancer: a case report. Arch Phys Med Rehabil. 2005;86(6):1258–1260. doi:10.1016/j.apmr.2004.11.024 [CrossRef]
- Miller EH, Benedict FE. Stretch of the femoral nerve in a dancer: a case report. J Bone Joint Surg Am. 1985;67(2):315–317. doi:10.2106/00004623-198567020-00019 [CrossRef]