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Orthopedics and Sports Medicine Case of the Month

August 2012: Patella or Kneecap Instability

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Gregory Schmale 173x173

Dr. Gregory Schmale discusses the orthopedic management of an adolescent with an unstable patella, including the treatment alternatives and indications for surgery.

Patient History

Figure 1. A standing view of both lower limbs

Figure 1. A standing view of both lower limbs.

A 9-year-old boy presented to our Orthopedics and Sports Medicine Clinic with recurrent “giving way” of one knee. He was not sure what was wrong. Though he had had no obvious episodes of dislocation of the kneecap, he thought that at times maybe the kneecap was slipping to the side, causing him to nearly fall, then it would slip back in place again. The giving-way episodes were initially quite painful, though much less so as they became more frequent. The first time it happened, his knee swelled. Since that time, he has had little swelling with these events.

Figure 2. Two additional views of the knee suggesting the more outside or lateral position of the patella

Figure 2. Two additional views of the knee suggesting the more outside or lateral position of the patella.

In clinic, his exam was notable for apprehension, or a feeling like his left knee was giving out, when his patella was pushed to the outside while his knee was gently bent. Otherwise, he had an unremarkable exam, except his thigh muscle on the left, his quadriceps muscle, seemed smaller than on the right. On standing, he did not appear knock-kneed or bow-legged.

Figure 3. A sunrise view of the patella, revealing the more lateral and tipped position of the left patella

Figure 3. A sunrise view of the patella, revealing the more lateral and tipped position of the left patella.

Radiographs suggested a near-neutral alignment on the left, with the patella lying just lateral or outside to the ideal position (Figure 1). This matched his clinical appearance of a fairly straight limb. Additional films of the knee revealed a slight outside position (Figure 2) and lateral tilt (Figure 3) to the patella.

The diagnosis of patellar instability was discussed with the patient and family. The recommended treatment at this time was a conservative one: we recommended he wear a neoprene sleeve with a hole cut out for the patella when awake, and we prescribed exercises to strengthen the inner or medial quadriceps muscles to help hold the patella in place.

Anatomy and Function of the Patella and Patellofemoral Joint

. A three-dimensional computed tomography (CT) scan of a knee with a laterally lying patella at risk for dislocation

Figure 4. A three-dimensional computed tomography (CT) scan of a knee with a laterally lying patella at risk for dislocation. The blue and yellow lines (the medial or inner patella-femoral ligament) help hold the patella in place in the groove or trochlea at the end of the femur.

The patella lies within the tendon of the quadriceps muscle, the muscle that straightens the knee. Where it lies, the patella makes the effectiveness of the quadriceps greater, serving as a pulley for the quadriceps, giving the quadriceps better leverage to extend or straighten the knee. The patella is normally positioned over a groove or trough, known as the trochlea, at the end of the femur. The depth of the trochlea and the shape of the patella help hold the patella in place. Ligaments passing between the patella and femur help to hold the patella well centered over the trochlea (Figures 4 and 5). These ligaments are especially important stabilizers during the start of knee flexion, or bending of the knee, from a position of full extension or a straight knee, where the groove that the patella lies in is most shallow.

Figure 5. Three-dimensional CT scan of the same knee, with the view rotated

Figure 5. Three-dimensional CT scan of the same knee, with the view rotated so that the viewer is looking up from below the knee. The blue and yellow lines depict the stretched out medial or inner ligament, allowing the patella to slide off to the outside. When the patella is in this position, the outer ligaments and tissues (here shown in red) tend to contract or shorten, holding the patella in a poor position. This patient is also seen to have a very shallow groove or trochlea at the end of the femur.

When the patella dislocates, it typically slides to the outside, out of the groove. The patient experiences that “giving-way” feeling as the knee buckles. For a dislocation to occur, the medial or inner ligaments must be either stretched or torn, or the bone where they attach to the femur or patella must fracture. How this tissue heals, in a tight or loose fashion, plays a major role in whether or not the patella has a tendency to redislocate. However, numerous studies suggest that there is no advantage to surgically repairing torn or fractured structures for patients with first-time dislocation of the patella, that recovery in the long–term is equivalent or better with a conservative program of bracing followed by exercises.1,2,3 

Treatment for Our Patient

Figure 6. One technique for patellar stabilization described by Deie

Figure 6. One technique for patellar stabilization described by Deie (4): The patella is stabilized by reconstruction of the medial or inner ligament — the medial patellofemoral ligament, an important medial restraint to lateral movement of the patella. We use one of the hamstring tendons, left intact on its tibial attachment, freed from its muscle and passed through the posterior third of the medial collateral ligament, then up through a drill hole in the patella. This produces a leash or restraint, holding the patella in the best position.

The patient accepted the challenge of a rigorous home therapy program, but despite measurably increasing his quadriceps muscle size and strength with therapy, he had recurrent episodes of giving-way. This suggested that his inner thigh muscle was not going to provide the knee stability that he needed, that the inner ligaments were too stretched out to hold his patella in position.

The patient and family requested a re-evaluation for consideration of a surgery to help hold the patella in place. We re-evaluated the patient, established that his limb was not so knock-kneed that the patella was being pulled to the side by a crooked alignment across the knee and suggested a surgical treatment known as arthroscopy (inserting a small viewing device into the knee attached to a camera, about the dimensions of a pencil) to follow the position of the patella, releasing or lengthening the tight lateral tissues, and reconstruction of the inner or medial ligament using one of the patient’s own knee flexor tendons, one of the small hamstrings (Figure 6).

The surgery was performed on an outpatient basis. The patient was placed on crutches and instructed to put only half of his weight on the leg for the first six weeks. He then progressed to full weight-bearing, starting jogging at four months, running at five months, jumping at six months and at seven months took a test to establish safety for return to sports.


A patient who is still growing at the knee, who has open growth plates as this patient did, requires a surgery that does not interfere with further growth. Many adult treatments for patellar instability include cutting across the tibia (or leg bone) in a fashion that would injure the tibial growth plate in a growing child.

We were careful to establish that this patient’s limb was quite straight. If the limb is crooked, the patella may not track optimally. A patella riding on the outer edge of the groove at the end of the femur, because the leg is quite knock-kneed, is one major risk factor for patellar instability. Surgical treatment of such a crooked limb in a patient who is still growing could include temporary closure of the medial growth plate of the femur (thigh bone) at the knee to correct the alignment. Either suture tightening of the inner quadriceps to patella and femur or reconstruction of the medial patellofemoral ligament as described earlier would decrease the likelihood of recurrence.


Patellar instability is a challenging problem for the patient and the physician. The patella balances over the groove or trochlea at the end of the femur, being pulled inward by muscle and ligaments that are easily stretched with injury, and outward by muscles and ligaments that are quick to contract and shorten with healing. Keeping the inner thigh muscle, the medial quadriceps muscle, strong is a key first line in management of patellar instability. Braces with buttresses to push the patella inward can be a mechanical aid, as well as a good reminder to the brain where the knee is in space. When dislocations recur despite a rigorous strengthening program, surgery may be considered and can provide a solution to recurrent instability of the patella.


  1. Nikku R, Nietosvaara Y, Kallio PE, Aalto K, Michaelsson JE. Operative versus closed treatment of primary dislocation of the patella. Similar 2-yr results in 125 randomized patients. Acta Orthopaedica Scandinavica. 68(5):415-8, 1997.
  2. Nikku R, Nietosvaara Y, Aalto K, Kallio PE. Operative treatment of primary patellar dislocation does not improve medium-term outcome: A 7-year follow-up report and risk analysis of 127 randomized patients. Acta Orthopaedica. 76(5):699-704, 2005.
  3. Hilton RY, Sharma KM. Acute and recurrent patellar instability in the young athlete. Orthopaedic Clinics in North America. 34(3):385-96, 2003.
  4. Deie M, Ochi M, Sumen Y, Yasumoto M, Kobayashi K, Kimura H. Reconstruction of the medial patellofemoral ligament for the treatment of habitual or recurrent dislocation of the patella in children. Journal of Bone and Joint Surgery (Br). 85-B, (6):887-890, 2003.

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