Pearls and pitfalls for treating first-time and recurrent dislocations

What’s the most effective strategy for treating a young gymnast with a first-time patellar dislocation? How about a varsity soccer player with recurrent dislocations—what’s the best way to get him back in the game?

Donald C. Fithian, MD, addressed these questions and more during a symposium on treating patellofemoral instability in young athletes. He also discussed anatomic factors that can lead to patellar dislocation, treatment options for first-time and recurrent patellar dislocations, and technical pearls and pitfalls for surgery.

Anatomy and biomechanics
According to Dr. Fithian, the medial patellofemoral ligament (MPFL) is the most important soft-tissue stabilizer in the knee, followed by the medial patellomeniscal, the medial patellotibial, and the lateral patellofemoral ligaments.

“If you cut the MPFL of a cadaver, you can easily dislocate the patella,” he noted. “The only role of the MPFL is to resist lateral displacement of the patella, and it is ideally located.”

The trochlear groove, said Dr. Fithian, also plays a crucial role in stabilizing the knee. Studies conducted by leading French researchers, including Henri Dejour, MD, and Gilles Walch, MD, have supported the viewpoint that trochlear dysplasia is a fundamental factor leading to patellar dislocation.

“When the groove is deficient,” said Dr. Fithian, “as it is in many patients with patellofemoral instability, the retinacular ligaments take on a greater role.

“Even if the quality of the trochlea is not a problem,” he continued, “the medial ligaments must be abnormally lax for the patella to dislocate.”

Evaluating patients for trochlear dysplasia
A patient evaluation should include a thorough history, a physical examination, and lateral radiographs. Drs. Dejour and Walch first described a method for evaluating radiographs to find signs of trochlear dysplasia.

“Evaluate the height of the medial and lateral trochlear walls with respect to the depth of the groove using the lateral radiograph view,” said Dr. Fithian. “The ‘crossing sign,’ which indicates trochlear dysplasia, represents the point at which the bottom of the groove comes up and is co-linear with the facets.”

The prominence of the floor of the groove with respect to the anterior cortex of the distal femur should also be evaluated. A prominence greater than 4 mm indicates trochlear dysplasia.

Another test is the lateral offset of the tibial tuberosity versus the trochlea. “This is sometimes called the ‘Q angle’ but is better described as a tibial tuberosity vs. trochlear groove offset, or ‘TTTG,’ which is a more reproducible measure,” he said.

“You can do this on tracing paper or on a superimposed computed tomography (CT) scan,” explained Dr. Fithian. “If you’re using magnetic resonance imaging, the best way to do that—assuming it’s a digital image—is to scroll down to the tibial tuberosity and put your cursor there, come back up to the level of the trochlear groove, and draw a line to indicate lateral offset.”

He noted that a CT scan is also useful in measuring patellar tilt. “When you’re measuring on a CT scan, pay attention to how much flounce or slack is seen in these lateral structures,” he stated. “A loose lateral ligament, represented by flounce or slack on the axial images, will not tilt the patella. The tilt results from an imbalance due to MPFL laxity.”

First-time vs. recurrent dislocations
When determining a treatment strategy for patients with first-time dislocations, Dr. Fithian noted that surgeons should first look for joint surface and/or retinacular injury.

“The risk of recurrence develops directly as a result of retinacular injury, which is frequently found in patients with first-time dislocations. The question is whether the injury requires surgical repair,” he said.

Advocates of acute MPFL repair, he said, have not yet demonstrated that surgery yields improved outcomes compared to conservative measures, such as knee bracing and physical therapy.

“Four high-quality, prospective, randomized studies have compared surgical and conservative treatment after initial patellar dislocation,” stated Dr. Fithian. “None of these studies has found any benefit of immediate medial retinacular repair.”

In patients with episodic patellar dislocation, however, surgery is often the most effective strategy.

“Patients with recurrent patellar instability are much more likely to continue experiencing dislocations than patients who have had only one dislocation episode,” he said. “For recurrent patellar instability, surgical stabilization generally is recommended to reduce the risk of further dislocations.”

MPFL reconstruction
According to Dr. Fithian, MPFL laxity must be documented by physical examination and stress radiography before the surgeon performs MPFL reconstruction. Laxity may also be evaluated through arthrometry, in which a known load is applied and displacement is measured.

“If the patient is too apprehensive, due to pain, to allow an adequate examination, an examination under anesthesia and arthroscopy should routinely precede the reconstruction to document laxity objectively and identify cartilage lesions that may affect outcomes,” he said.

The ideal candidate for isolated MPFL reconstruction has little pain between episodes of patellar instability. “Patients should be counseled ahead of time that stabilization of the patella will not reliably treat any pain that is not directly caused by their brief instability episodes,” he noted.

According to Dr. Fithian, MPFL reconstruction has two main surgical objectives:

“First, we re-establish the natural check-rein against lateral patellar motion. This check-rein, which is a palpable, firm stop when the patella is passively displaced in a lateral direction, represents the tethering of the patella by the intact MPFL.

“Second, we re-establish normal limits of passive lateral patellar motion. The goal is to restore passive stability to the patella so that it is free to glide laterally up to 9 mm, at which time the reconstructed medial ligamentous tether will engage to prevent further displacement,” he explained.

After the graft is fixed in place, the surgeon should confirm that the patella is not overconstrained.

“The patella should enter the trochlea from the lateral side,” he said. “At 30 degrees of knee flexion, the patella should be centered or in a slightly lateral position. When the patella is manually displaced, the graft should tighten to prevent excessive mobility.

“Once the graft is fixed to both the patella and femur,” he continued, “there should be a good endpoint, or check-rein, with the knee in full extension and at 30 degrees of flexion. Knee range of motion should be full, and lateral patellar displacement from the centered position at 30 degrees of flexion should be between 7 and 9 mm.”

Placement of the femoral attachment, he said, is one of the most critical steps in the procedure.

“The surgeon should ensure the graft works smoothly in early flexion, in concert with the trochlea,” he advised. “Adjust the pin site to minimize length change with knee flexion. If lengthening occurs in flexion, replace the pin more distally toward the epicondyle. If lengthening occurs in extension, move the pin more proximally toward the adductor tubercle.”

Tibial tubercle osteotomy
Patients with trochlear dysplasia may have evidence of accelerated wear on the cartilage of the lateral patella and trochlea.

“In such cases,” stated Dr. Fithian, “additional treatment may be considered to unload the cartilage in those areas. Displacement osteotomy of the tibial tubercle is effective for offloading specific areas of contact between the patella and femur.”

Dr. Fithian acknowledged the controversy over whether knee extensor realignment, such as lateral release and/or medialization of the tibial tuberosity, should be done routinely for patients with recurrent patellar instability.

“Neither tubercle medialization nor lateral retinacular release leads to greater patellar constraint,” he said. “These procedures should, therefore, be directed to relieving excessive lateral facet pressure, unloading arthritic cartilage, or relieving tension in the lateral retinaculum.”

Rehabilitation
Dr. Fithian stated that pain management after MPFL reconstruction is generally easier than for other reconstructive surgeries of the knee, possibly because the synovium is not disturbed during surgery.

“A femoral nerve block may be used intra-operatively, and oral narcotics may be prescribed when the patient is discharged,” he explained.

Although full weight-bearing is allowed immediately, a drop-lock or knee extension brace should be used for up to 6 weeks.

“Physical therapy is needed to restore quadriceps control and range of motion as soon as possible. If the patient does not have at least 90 degrees of flexion by 6 weeks, the intensity of the therapy program should be increased.”

Finally, he noted, “manipulation under anesthesia may be needed between 9 and 12 weeks after surgery if stiffness does not resolve with therapy alone.”

Bottom line

• Trochlear dysplasia is a fundamental factor leading to patellar dislocation.
• Surgical stabilization is generally recommended for recurrent patellar instability.
• An intense physical therapy program is recommended to restore quadriceps control and range of motion after surgery.

Jennie McKee is a staff writer for AAOS Now. She can be reached at mckee@aaos.org

References

1. Dejour H, Walch G, Neyret P, et al: Dysplasia of the femoral trochlea. Rev Chir Orthop Reparatrice Appar Mot 1990; 76:45-54.
2. Bassett FH: Acute dislocation of the patella, osteochondral fractures, and injuries to the extensor mechanism of the knee. In: Burke E, ed. Amer Acad Ortho Surg Instructional Course Lectures. St. Louis, MO: C.V. Mosby, Inc; 1976: 40-49.
3. Fithian DC, Neyrey P, and Servien E: Patellar instability: The Lyon Experience. Tech Knee Surg 6(2): 112-123.