Medial patella luxation treatment
Dr Abbie Tipler ATCL BVSc MANZCVS (Surgery) Veterinary Specialist Services 1-15 Lexington Road, Underwood, QLD 4119
Aetiology For a patella to function correctly it needs a proper alignment to the quadriceps muscle group that meets and converges on the patella. With correct alignment, the patella sits in the trochlear groove and this groove develops normally over time. The entire mechanism from pelvis to quadriceps to patella to tibial tuberosity should sit in a relatively straight line. This mechanism is called the extensor apparatus and when aligned with the patella it works to effectively and painlessly extend the knee via contraction of the quadriceps. There can be several facts that can contribute to the abnormal development that is seen in dogs with medial patella luxation (MPL). Puppies are born with normal stifles, however medial malalignment of the quadriceps mechanism leads to abnormal forces on the growth plates during growth and conformational abnormalities result. The conformational abnormalities can typically involve varying degrees of torsion of the tibia with internal rotation, distal femoral varus, hypoplasia of the medial condyle, patella alta and underdevelopment of the trochlear groove.
Signalment This is primarily a disease of small breeds such as Chihuahuas, Toy Poodles, Yorkies, Silky Terriers, Papillons and Pomeranians but is also seen in larger breeds such as Labradors, Staffordshires and Bull Terriers. Females tend to be over-represented and it is heritable. Around 50%-60% of the time it is bilateral. Around 80% of luxations are either a grade 2 or a grade 3. Around 10-40% of dogs have a concurrent cruciate ligament rupture. MPL has also been associated with trauma resulting in angular limb deformities, surgical repair of cruciate ligament disease or fractures/fracture repair, so take a thorough history. True cases of traumatic MPL are fairly rare.
Diagnosis Gait and stance examination – typically see a skipping lameness, stance may or may not be ‘crouched’ if the patient cannot effectively extend their stifles. Musculoskeletal examination focusing on grading the luxation and checking for concurrent instability of the stifle that may be associated with cruciate ligament disease. Grading: Grade 1 – 4. Grade 1 – patella can be luxated but immediately returns. Grade 2 – moves more freely in and out of the trochlear groove. When it is manually luxated, it does not immediately return to groove. Grade 3 – patella generally sits medial to the trochlear groove but can be manually replaced. Grade 4 – patella sits medially to the trochlear groove and cannot be manually replaced.
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378 Radiographs to examine for bony conformational abnormalities. Radiographic technique is important and straight views should be obtained. CT is sometimes required in cases with significant torsion.
Treatment Recommended for all lame dogs and young dogs with grade 2 luxations and above to prevent pain and lameness later in life. Early treatment is recommended to prevent secondary cartilage erosion and osteoarthritis. Treatment depends on grade, degree of bony abnormalities, patient size and in some cases clinical factors such as patient temperament. A proportion of cases, especially grade 4 luxations or in situations where there is significant femoral varus, more advanced procedures are required and these have a higher rate of complications. This summary discusses the traditional techniques. Step 1 – Careful examination of the joint for any signs of cruciate ligament disease, osteoarthritis, cartilage damage to either the patella or trochlear ridges and trochlear ridge depth is the first step. Step 2 - In all but very rare circumstances, such as a traumatic luxation with a normally formed groove, we will perform a trochleoplasty. This is a deepening of the trochlear groove and the goal is to achieve a trochlear groove that is deep enough and wide enough that approximately 50% or less of the patella protrudes above the trochlear ridges. Performing a trochleoplasty reduces the risk of re-luxation and without a trochleoplasty you have around a 19% re-luxation rate. You do however get more osteoarthritis with a trochleoplasty and it should be performed carefully with minimal damage to the cartilage. There are two common techniques – a wedge trochleoplasty and a block trochleoplasty. I prefer the wedge and I focus it proximally to ensure it is deep enough such that the patella is within the groove in full extension. Step 3 – Realignment of the quadriceps mechanism by lateralisation of the tibial tuberosity. This is called a tibial tuberosity transposition. The osteotomy should not be greater than one third the width of the tibial plateau and must avoid the meniscal cartilages. It is ideal to leave the distal periosteum intact as this acts as a tension band. K-wires in the tibial crest should be directed slightly medially and proximally. Two k-wires is more secure and the placement of a tension band wire further increases security. Step 4 – Soft tissue techniques, broadly speaking, tighten the soft tissues on the lateral side and release soft tissues on the medial side. Given tissues can stretch and repair, they should not be relied upon exclusively. In my experience, when you are needing to rely on soft tissue techniques to hold the patella in place, you generally have not addressed the bony deformities adequately. The only exception to this is for cases of traumatic luxation where there are truly no bony abnormalities (rare) and potentially for young animals to help correct during growth and in these cases we may place an anti-rotational suture. Soft tissue techniques involve medial release (medial retinaculum, medial joint capsule, sartorius, rectus femoris muscle) and lateral fascial overlap.
Post-operative Care
Proceedings of VetFest 2020 Tipler, A – Medial patella luxation treatment
379 Postoperative care consists of confinement to a small room or pen with leash walks to the toilet 3-4 x a day, possible placement of a light dressing for 2-3 days to cover the wound, analgesia, physiotherapy and follow-up radiographs at 6 weeks to check osteotomy healing.
Complications Reported at around 20% in most studies and up to 40% in others. - Re-luxation - Tibial crest avulsion - Patella tendon laceration - Unsatisfactory outcome - Pin migration - Pin irritation - Infection - Fractures of the trochlear ridge - Cartilage damage or dislodgement during trochleoplasty - There is no increased risk of complications between staged unilateral and bilateral procedures.
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