Pathophysiology of Medial Patellar Luxation (MPL)

MPL is the result of structural abnormality and progressive degradation in the bony and soft tissues of the pelvic limb. In skeletally immature animals with MPL, the displaced quadriceps group creates a restrictive "bowstring effect" causing abnormal angular and torsional femoral growth; the femoral trochlea does not develop normally because of the absence of normal patello-femoral compression. MPL is classified according to severity on a 4-point scale (Table 1).

Table 1. Patellar luxation classification.

Clinical Signs

Grade 1 MPL is primarily an examination finding in asymptomatic dogs. Dogs with grade 2 MPL often display episodic non-weight-bearing lameness. Repeated episodes of luxation/reduction erode the medial trochlear ridge causing progression to grade 3. Dogs with grade 3 or 4 MPL often have more persistent, yet less obvious lameness. Often these dogs are bowlegged and walk in a "crouched" gait due to inability to fully extend the stifles and are unwilling to jump onto furniture or ascend stairs. Some dogs with MPL are asymptomatic regardless of their grade 1-4 status. A history of mildly symptomatic MPL followed by a sudden worsening of lameness often indicates cruciate ligament rupture.

Treatment

Successful treatment of MPL requires detection and treatment of all skeletal and soft tissue pathology (Table 2).

Table 2. Treatment algorithm.

Skeletal reconstructions include trochleoplasty and tibial crest transposition. Surgical restoration of proper trochlear depth and length is commonly needed. Wedge or Block recession trochleoplasty techniques preserve the articular cartilage of the trochlea. Trochleoplasty disrupts the distal femoral physis.

Tibial crest transposition is indicated when the tibial crest is not properly aligned between the reduced patella and the central axis of the hock and pes. A caudally positioned osteotomy (large tibial tubercle fragment) can be performed in an oblique caudo-medial to cranio-lateral direction to allow cranial advancement of the tibial tubercle as it is laterally transposed. This slight cranial shift in the tibial tubercle decreases retropatellar pressure and may benefit patients with retropatellar chondromalacia. Preservation of the cortex and periosteum at the distal extent of the osteotomy allows the tibial tubercle to pivot while reducing the need tension band fixation. The tibial tubercle is stabilized with 2 Kirschner wires with the most proximal wire placed through the fibers of patellar ligament to assure purchase of the strongest portion of the tubercle. A tension band wire is placed if indicated (active dog, bilateral repairs, muscular dog, poor compliance anticipated, cats prone to jumping during convalescence, etc).

Soft tissue reconstructions include capsular / retinacular / muscular release, imbrications, and anti-rotation sutures. Soft tissue reconstructions, by themselves, will not correct bony conformational abnormalities. Release of thickened and contracted medial joint capsule/retinaculum is achieved by their incision from the tibial plateau to the suprapatellar recess. If needed, the quadriceps muscle group can be elevated from the suprapatellar region to the proximal femur, being careful to protect the descending genicular vessels. Stretched lateral joint capsule/retinaculum often need to be tightened to achieve balanced soft tension upon the patella. Extracapsular lateral fabello-tibial anti-rotation sutures can be placed to limit excessive stifle rotation and are particularly beneficial in dogs with combined MPL and cranial cruciate ligament rupture.

Mechanical Alignment Theory

It is likely that normal patellar stability is the result of both healthy static and dynamic constraints (such as proper alignment of the quadriceps/patellar mechanism with the underlying skeleton).¹ The mechanical alignment theory states that proper anatomic alignment of the quadriceps/patellar mechanism with the underlying skeleton promotes patellar stability. When the quadriceps muscle contracts, it seeks to form a straight line with the patella positioned between its origin and insertion. If the long axis of the quadriceps is not centered over the trochlear sulcus, there is an imbalance in muscular forces favoring patellar luxation (PL). Excessive femoral varus and external femoral torsion are commonly associated with medial patellar luxation (MPL). Distal femoral varus moves the long axis of the quadriceps muscle medial to the trochlear sulcus. This discrepancy causes a strong medial tensile force upon the patella during muscular contraction. The strength and length of the quadriceps muscle in large breed dogs amplifies this discrepancy as compared to small breeds of dogs. In instances of healthy static constraint, minor malalignment in the dynamic constraints may be tolerated without gross events of patellar luxation. However, when the static constraints are pathologic the role of malalignment in the dynamic patello-femoral constraint may be more critical. Corrective femoral osteotomy has been effectively used to treat MPL complicated by "excessive" femoral varus.¹

Femoral Varus Measurement

Femoral varus has been evaluated by measurement of anatomic lateral distal femoral angle (a-LDFA).⁶ The anatomic proximal femoral axis is determined by drawing a line through center points of the proximal femoral diaphysis; these points are identified at one-third and one-half of the femoral length with the a-LDFA method. Next the distal joint reference line is determined. The a-LDFA is the angle formed between the anatomic femoral axis and the lateral end of the distal joint reference line. The a-LDFA's measured from radiographs of Labrador Retrievers, Golden Retrievers, German Shepherds and Rottweilers were reported as 97°, 97°, 94° and 98° respectively, but the accuracy of these values in predicting the true anatomic conformation was not a component of that study.⁶ Multiple properly positioned radiographs are advised to ensure repeatability of measurements because incomplete hip extension and/or external rotation of the hips commonly induce dramatic artifactual distal femoral varus measurement. Femoral torsion can be measured from an axial radiographic view.

Corrective Femoral Osteotomy for Combined Femoral Varus & MPL

Most veterinary surgeons agree that when coexistent with MPL, excessive femoral varus should be corrected, however, an objective definition for "excessive" has not been established. Based upon the studies reported above, I currently advise corrective osteotomy when MPL is accompanied by a-LDFA > 102°. Long-term follow-up evaluation (4+ years) of large breed dogs treated with corrective femoral osteotomy for combined femoral varus and MPL had predictable femoral osteotomy healing, patellar stabilization, and long-term improvement in 10 parameters of patient comfort and function.¹

References

1.  JAVMA 231:1070-75, 2007

2.  Vet Surg 30:552-58, 2001

3.  Vet Surg 37: 43-48, 2008

4.  Vet Radiol Ultrasound 47:546-552, 2006.

5.  Vet Surg 36: 593-598, 2007.