Surgical Correction of Patellar Luxation in Cats
Caleb Hudson, DVM, MS, Diplomate ACVS
Patellar luxation is a relatively uncommon cause of hindlimb lameness in cats. Many clinically normal cats have mild femoropatellar joint laxity, and during orthopedic examination patellar subluxation or even low-grade luxation can often be induced.1 It is important to differentiate these clinically insignificant cases of patellar laxity from cases in which patellar luxation causes hindlimb lameness (Table 1).
Medial patellar luxation is more common in cats than lateral patellar luxation.3 In approximately 80% of cats, patellar luxation is bilateral, and its origin can be congenital or traumatic.2,4 In cats with persistent clinical lameness attributable to patellar luxation, surgical correction is recommended.3
During initial orthopedic examination, categorize the severity of the patellar luxation. A slightly modified 4-level grading scheme, originally developed to categorize patellar luxation in dogs, is commonly applied to cats (Table 2).4—6
|Table 2. Grading Patellar Luxation in Cats|
|GRADE||DESCRIPTION OF PATELLA|
Before surgical correction of patellar luxation, obtain mediolateral and craniocaudal projection radiographic views of the affected stifle joint. Evaluate the radiographs for the presence or absence of degenerative changes and joint effusion, as well as the shape and size of the tibial tuberosity (Figure 1).
If desired, obtain a skyline view of the trochlear groove, which allows trochlear groove depth to be assessed before surgery (Figure 2).2 The trochlear groove is an appropriate depth if 50% or more of the patella sits below the trochlear ridges.7
Read, published in the March/April 2012 issue of Today’s Veterinary Practice and available at , for further information about imaging the stifle joint.
The surgical approach to the stifle joint for correction of patellar luxation is through either a lateral or a medial incision, as described by Piermattei and Johnson.8 I recommend the:
- Lateral incision for correction of medial patellar luxation
- Medial incision for correction of lateral patellar luxation.
Position the patient in dorsal recumbency for either approach, with the affected limb clipped, prepped, and positioned off the end of the operating table (Figure 3A). A V-top surgery table is invaluable for maintaining the patient in dorsal recumbency during the operation.
Create a skin incision from the level of the tibial tuberosity proximal to the patella and then continue proximally a distance equal to the distance between the patella and tibial tuberosity (Figure 3B). Make the skin incision several millimeters lateral or medial to the patellar ligament and patella for the lateral or medial approach, respectively. Continue the skin incision through the subcutaneous tissue to the level of the fascia lata (laterally) or the medial femoral fascia (medially).
Incise the fascia on the same line as the skin incision, taking care to stay several millimeters medial or lateral to the patellar ligament and patella (Figure 3C). Proximal to the patella, the fascial incision can be angled slightly caudally to avoid the vastus lateralis muscle (lateral approach) or cranial part of the sartorius muscle (medial approach). Carefully dissect the fascia off the underlying joint capsule for several millimeters cranial and caudal to the fascial incision.
Incise the joint capsule and parapatellar fibrocartilage on the same line as the fascial incision, starting with a stab incision near the tibial tuberosity and continuing the incision proximally with blunt-tipped scissors to minimize the chance of damaging the underlying articular cartilage (Figure 3D).
Once the arthrotomy is complete, luxate the patella contralateral to the side on which the arthrotomy was performed, and evaluate the:
- Articular cartilage surfaces of the patella and trochlear groove for wear
- Depth of the trochlear groove relative to the height of the patella
- Cranial and caudal cruciate ligaments and cranial pole of the medial and lateral menisci for damage.
With use of an open arthrotomy approach, if the cranial cruciate ligament is intact, a thorough evaluation of the caudal pole of the medial and lateral menisci is very difficult in the feline stifle.
In patients with a shallow trochlear groove (Figure 4A, see SURGICAL APPROACH, Step 5. Evaluation of the Joint), a trochleoplasty procedure is recommended to deepen the groove. The block recession is my preferred technique because it maximizes articular cartilage contact between the patella and trochlear groove, particularly near the proximal and distal ends of the groove.9
To perform the block recession:
- Use a fine-tooth hobby saw (X-ACTO blade; xacto .com) to create 2 abaxial osteotomies just axial to the highest point on the trochlear ridges (Figure 4B). Angle the osteotomies 10° axially, relative to the sagittal femoral plane. Osteotomy depth should allow the osteotomies to extend from the proximal to the distal end of the trochlear groove (Figure 4C).
- Use a fine osteotome or Beaver scalpel blade (#376400 or #374562) to remove the block of trochlear bone and articular cartilage defined by the abaxial osteotomies. Intermittently advance the osteotome or Beaver blade from both the proximal and distal ends of the block to minimize the chance of fracturing the trochlear bone segment.
- Once the block of bone and cartilage is removed (Figures 4D and 4E), deepen the abaxial osteotomies an appropriate amount (1—2 mm of bone in most cats) to allow ≥ 50% of the patella to sit in the trochlear groove once the block of trochlear bone and cartilage is replaced. Evenly resect the subchondral bone base of the recipient trochlear site by using a fine osteotome, Beaver blade (#376400 or #374562), or a narrow bone rasp.
- Replace the previously removed block of trochlear articular cartilage and subchondral bone and press-fit it into the recipient bed using firm finger pressure or a smooth-handled instrument (Figure 4F).10
Soft tissue techniques alter parapatellar soft tissue tension to help maintain the patella in the trochlear groove. In most cases of patellar luxation, perform these techniques as part of the correction, but not as the sole correction, except in cases of traumatic patellar luxation. Techniques include:
- Release of contracted parapatellar soft tissues on the side toward which the patella is luxated
- Tightening (imbrication or partial resection) of stretched parapatellar soft tissues opposite the direction of patellar luxation.
To perform a retinacular release:
- Incise the contracted parapatellar soft tissue structures, including muscle fascia or joint capsule, starting several millimeters above the tibial tuberosity.
- Extend the incision proximally to the patella, staying 2 to 4 mm away from the patellar ligament, and continue it for an equal distance proximal to the patella. If necessary to relieve tension on the patella, continue the desmotomy incision even farther proximally.
- In less severe cases of soft tissue contraction, release of the joint capsule and parapatellar fibrocartilage is usually sufficient, while in more severe cases, the muscle fascia should be released as well.
- Leave the released tissues unsutured to help minimize the redevelopment of contraction during the healing period.
To reduce parapatellar soft tissue laxity in the joint capsule and muscle fascia opposite the direction of luxation, perform a combination of tissue resection and imbrication. I usually reduce soft tissue laxity while closing my arthrotomy, because I perform the arthrotomy on the side of the stifle on which laxity should be reduced (lateral for a medial luxation and medial for a lateral luxation). To perform this technique:
- Excise a section of joint capsule starting just above the tibial tuberosity and continuing proximal to the level of the patella; then extending an equal distance proximal to the patella. The amount of joint capsule removed corresponds to the amount of excessive laxity I believe to be present.
- Appose the joint capsule edges by using an interrupted or cruciate mattress suture pattern of monofilament, long-lasting absorbable suture material.
- To eliminate muscle fascia laxity, imbricate the muscle fascia by overlapping the fascial edges using a modified Mayo mattress (vest over pants) suture pattern with monofilament, long-lasting absorbable or nonabsorbable suture material (Figure 5). The amount of overlap varies from patient to patient and should be sufficient to remove laxity from the parapatellar soft tissues on the side on which the imbrication is performed while the patella is reduced (typically 3—6 mm of fascial overlap).
Tibial tuberosity transposition is a surgical technique that moves the attachment of the patellar ligament medially or laterally on the proximal tibia to help keep the patella aligned in the trochlear groove.
To perform a tibial tuberosity transposition:
- Perform an osteotomy to free the tibial tuberosity and crest, using a sharp osteotome or a sagittal saw. Begin the osteotomy caudal to the attachment of the patellar ligament on the tibial tuberosity but cranial to the intermeniscal ligament, and then extend it distally and end at the distal end of the tibial crest. The cortical bone on the cranial aspect of the tibia at the distal end of the crest and the associated fascial attachments should be left intact (Figure 6A).
- Shift the proximal end of the tibial tuberosity segment medially (for a lateral luxation) or laterally (for a medial luxation), allowing the origin of the quadriceps femoris muscle, the patella and trochlear groove, and the attachment of the patellar ligament to form a straight line (typically 1—3 mm).
- Fix the proximal end of the tibial tuberosity in place by using 2 Kirschner wires inserted through the tibial tuberosity just distal to the attachment of the patellar ligament (Figures 6B and 6C).
- Orient the Kirschner wires perpendicular to the long axis of the tibia to prevent accidentally penetrating the stifle joint, and angle them slightly axially in the sagittal plane to ensure good purchase in the tibia.
- Advance the pins until they are seated in the caudal cortex of the proximal tibial metaphysis, cut off the protruding ends of the Kirschner wires close to the tibial tuberosity, and countersink or bend them over to stay close to the bone (Figure 6D).
- If the cortical bone at the distal end of the tibial crest is transected during the osteotomy, place a tension band using 22- or 24-gauge orthopedic wire in a “figure 8” pattern around the Kirschner wires and through a bone tunnel in the tibial diaphysis distal to the tibial crest.
SURGICAL DECISION MAKING
Choosing which of the described surgical techniques to use in a given patient can be difficult. Most patients do not need to have all 4 of the described correction techniques performed in order to achieve appropriate patellar tracking without luxation. My approach is to:
- Assess the depth of the trochlear groove: If the groove is shallow (see SURGICAL APPROACH, Step 5. Evaluation of the Joint), the first correction I perform is to deepen the groove by using a block recession.
- After the trochleoplasty has been performed, reduce the patella into the trochlear groove and put the stifle through range of motion. If the patella cannot be reduced because of soft tissue tension, or if it reduces but easily reluxates, perform a retinacular release.
- Once the release is performed, try again to reduce the patella. Assess patellar tracking during stifle flexion and extension, and with tibial internal rotation. If the patella still luxates, perform a retinacular tightening.
- After tightening is performed, repeat the stifle range of motion. If the patella will not luxate, the procedure is finished; however, if the patella still luxates, the last surgical correction to perform is a tibial tuberosity transposition.
Obtain craniocaudal and mediolateral projection radiographs of the stifle joint (Figure 7) immediately post-operatively and at the 6- and 12-week recheck appointments. The radiographs should be critically evaluated, specifically:
- Assessing the location of the patella and position of any implants
- Evaluating the tibial osteotomy site for evidence of bone healing if a tibial tuberosity transposition has been performed.
I typically do not place a bandage on the hindlimb, although I do cover the skin incision with an adhesive wound dressing, which I leave in place for the first 3 to 5 days after surgery.
- Provide analgesia with injectable IV buprenorphine boluses, 0.03 mg/kg Q 6 H, for the first 12 hours.
- After 12 hours I switch to sublingual buprenorphine, 0.01 mg/kg Q 8 H.
- Dispense sublingual buprenorphine, 0.01 mg/kg Q 8 H, for 10 days after surgery.
- Treat the incision with cold pack therapy, 10 minutes Q 6 H, for 2 to 3 treatments.
Patients are typically kept in the hospital overnight and discharged the day after surgery. Postoperative activity is limited to strict crate rest for 8 weeks after surgery. Postsurgery recheck examinations are performed at 2, 6, and 12 weeks.
Patellar luxation can cause clinically significant hindlimb lameness in cats. Patients with persistent lameness attributable to patellar luxation should be considered candidates for surgical therapy.
Surgical treatment of patellar luxation is typically a multistep process. The exact techniques required vary from patient to patient. The final decision about which surgical techniques to use in a particular patient is based on intraoperative assessment of patellar tracking in the trochlear groove.
Use of sound decision making and intraoperative attention to detail lead to consistently successful surgical correction of feline patellar luxation.
- Smith GK, Langenbach A, Green PA, et al. Evaluation of the association between medial patellar luxation and hip dysplasia in cats. JAVMA 1999; 215(1):40-45.
- Scott HW, McLaughlin R. Feline Orthopedics. London: Manson Publishing Ltd, 2007, pp 218-222.
- Johnson ME. Feline patellar luxation: A retrospective case study. JAAHA 1986; 22:835-838.
- Loughin CA, Kerwin SC, Hosgood G, et al. Clinical signs and results of treatment in cats with patellar luxation: 42 cases (1992-2002). JAVMA 2006; 228(9):1370-1375.
- Putnam RW. Master’s thesis: Patellar luxation in the dog. Ontario, Canada: University of Guelph, 1968.
- Singleton WB. The surgical correction of stifle deformities in the dog. J Small Anim Pract 1969; 10(2):59-69.
- Kowaleski MP, Boudrieau RJ, Pozzi A. Stifle joint. In Tobias KM, Johnston SA (eds): Veterinary Surgery: Small Animal. St. Louis: Elsevier, 2012, pp 906-998.
- Piermattei DL, Johnson KA. An Atlas of Surgical Approaches to the Bones and Joints of the Dog and Cat, 4th ed. Philadelphia: Elsevier Saunders, 2004, pp 342-349.
- Talcott KW, Goring RL, de Haan JJ. Rectangular recession trochleoplasty for treatment of patellar luxation in dogs and cats. Vet Compar Orthop Traumatol 2000; 13:39-43.
- Piermattei DL, Flo GL, DeCamp CE. Handbook of Small Animal Orthopedics and Fracture Repair, 4th ed. St. Louis: Saunders, 2006, pp 562-632.