Placement of a Canine Mini-trochlear Prosthesis in a Cat with Chronic Patellar Luxation

Abstract

To report the placement of a trochlear groove prosthesis in a domestic cat with grade 4 medial patellar luxation and severe patellofemoral osteoarthritis using a mini-implant designed for dogs. A 1-year and 9-month-old female cat with left pelvic limb 3/6 grade lameness of 10-month duration was attended at a private veterinary hospital. Physical examination showed marked periarticular thickening, reduced femoral muscle mass, and stifle crepitation. Radiographic changes were consistent with joint effusion, decreased joint space between femoral and tibial medial condyles, and osteophyte on the distal patellar margin. Trochleoplasty was not recommended due to articular deterioration, and the placement of a canine #3 trochlear prosthesis was planned. The base plate was fixed with cortical screws and the trochlear prosthesis was attached through Morse taper abutments. Radiographic follow-ups confirmed the adequate position of both the patella and trochlear prosthesis. After 30 days, the cat presented 1/6 grade lameness and was medically discharged after 60 days. After 2 years, the cat presented normal mobility using a mini-canine trochlear prosthesis. Using a trochlear prosthesis to replace the femoral patellar groove of cats is suggested as a viable strategy to align the extensor mechanism, improve patellar stability, and manage patellofemoral osteoarthritis-induced lameness.

Keywords

patellar luxation - patellar dislocation - trochlear prosthesis - patellar groove - cat

Introduction

Medial and bilateral patellar are the most common luxations in cats. Although it can be considered a developmental condition, the role of trauma as a cause is still under debate.[1] Clinical signs of patellar luxation in cats greatly vary from crouching gait, jumping reluctance, bow-legged conformation, and intermittent locking during stifle extension up to severe joint wear and consequent release of inflammatory mediators. Patellar luxation and lameness may only be intermittent.[2]

The treatment of luxation aims to neutralize the forces that tend to dislocate the patella from the femoral trochlear groove. Conventional techniques such as femoral trochleoplasty, tibial tuberosity transposition, medial desmotomy or lateral imbrication of the joint capsule, partial release of rectus femoris and vastus medialis muscles, anti-rotational suture, reinforcement of the lateral retinaculum with fascia lata graft, distal femoral corrective osteotomy, and partial parasagittal patellectomy may be inefficient and eventually perpetuate osteoarthritis and maintain chronic pain. Although repositioning the patella on a degraded surface improves joint mechanics, continuous friction and wear lead to subchondral bone exposure and undermine the animal's quality of life. Furthermore, it is widely recognized that joint pain in cats can be difficult to control.[3] [4] [5] [6]

Recently, Jaworski et al reported for the first time the successful use of a trochlear prosthesis in a domestic cat with chronic grade 4 medial patellar luxation and severe patellofemoral osteoarthritis.[7] Considering the limited experience of trochlear augmentation in cats and the unavailability of trochlear grove implants for cats, this case report describes the placement of a trochlear prosthesis designed for small dogs in a cat with chronic medial patellar luxation.

Case Description

A 1-year and 9-month-old, female, neutered, mixed breed, domestic cat, weighing 3.25 kg, with a normal 5/9 body condition score, and left pelvic limb 3/6 grade lameness of 10-month duration of apparently non-traumatic origin was attended at the reporting institution.[8] [9] Marked periarticular thickening of the left stifle, moderate reduction in femoral muscle mass, medial patellar dislocation, and pain during manipulation were evident. Both cranial drawer and tibial compression tests were negative.

Standard radiographs revealed medial dislocation of the patella, reduced size of infrapatellar fat pad consistent with joint effusion, decreased joint space between the medial condyles of the femur and tibia, and presence of osteophytes at the distal margin of the patella ([Fig. 1]). Despite the chronicity of the case, the distal femoral alignment was normal without varus or valgus deviation. Both anatomical medial distal femoral angle (aMDFA) and anatomical lateral distal femoral angle (aLDFA) were within normal ranges.[10] [11] The mechanical axis of the femur and the aLDFA showed no angular or torsional deviations or deformities.

Fig. 1 Preoperative cranio-caudal (A) and medio-lateral (B) radiographs of the cat's left stifle. The red dotted circle indicates the medial patellar luxation and the small arrow the erosion of the medial trochlear crest, marked flattening of the femoral trochlea, and ectopic pseudo-trochlear groove.

Trochleoplasty was not recommended due to apparent deterioration of the articular surface, the severity of the femoral trochlear malformation, and secondary osteoarthritic alterations; thus, the prosthetic replacement of the patellar groove was determined as the most suitable treatment. The radiographs were imported to a surgery planning tool (https://vpop-pro.com) to measure the depth of the trochlear crests. The placement of a two-component canine #3 trochlear prosthesis (Lincevet, Rio Claro, SP, Brazil; https://lincevet.com.br/), which has a perforated grade 4 titanium base plate and a trochlea-shaped prosthesis made of grade 5 titanium (Ti6Al4V), was planned. The anatomically shaped top surface is highly polished and coated with diamond-like amorphous carbon with a very low friction coefficient and high scratching resistance.

After the use of pre-anesthetic medication (methadone combined with acepromazine) and anesthetic induction (ketamine combined with propofol), the cat was submitted to intravenous antibiotic prophylaxis, endotracheal intubation, and remained connected to the Baraka anesthesia circuit. Then femoral nerve locoregional blocks were performed. Anesthesia was maintained throughout the continuous infusion of propofol combined with remifentanil, ketamine, and dexmedetomidine.

A medial parapatellar arthrotomy with retinacular release and desmotomy of the medial patellofemoral ligament revealed that a pseudo-trochlear groove was evident medial to the medial portion of the trochlear ridge ([Fig. 2]).

Fig. 2 Intraoperative image of the femoral trochlea of the cat's left stifle. Medial patellar luxation, marked flattening of the femoral trochlea, ectopic pseudo-trochlear groove (blue arrow), and osteoarthritis with periarticular osteophyte can be seen.

The stifle was placed at skyline view position and the approximate trochlear groove depth was determined as 1.5 mm with the aid of a 1-mm-scaled Castroviejo-type caliper. One tip of the caliper was placed on the deepest surface of the trochlear groove, while the other tip was placed in contact with a ruler leaned on the trochlear crests. A single and precise linear cut was performed from the digital extensor tendon to the end of the cartilage with a 0.3-mm-thick blade (20 × 40 mm) mounted on a micro oscillating saw (NS-2012, Ruijin, Wuhu, China) under irrigation.

The base plate was fixed with two 1.5-mm-diameter cortical screws. Then the three Morse taper abutments of the trochlear prosthesis were positioned onto the base plate. Next, a specific pin impactor was positioned at the center of the trochlear prosthesis and firmly tapped with an orthopedic hammer. The 1-mm-thickness gap between the base plate and the trochlear prosthesis indicated that the Morse taper abutments closely attached both components.

The joint capsule and retinacular tissues were closed in simple appositional pattern with polydioxanone monofilament sutures (2-0, Atramat, Mexico DF, Mexico). The adequacy of patellar stability was determined with passive flexion, extension, adduction, abduction, and twisting manipulation movements. Superficial tissues were then routinely closed in layers. The intravenous administration of cephalothin was used as postoperative antibiotic prophylaxis. Radiographic images with the stifle in flexion and extension were taken before recovery from anesthesia to confirm the adequate position of both the patella and trochlear prosthesis, as well as the re-establishment of the femoral–tibia–patellar axis ([Fig. 3]).

Fig. 3 Postoperative cranio-caudal (A) and medio-lateral (B C) radiographs of the cat's left stifle. Observe the re-establishment of the femoral–tibia–patellar axis and adequate positioning of the patella and the trochlear prosthesis in extension and flexion.

The cat received oral antibiotics (amoxicillin + clavulanate) for 7 days, anti-inflammatory drugs for 4 days, and analgesics (tramadol hydrochloride) for 3 days. Although the cat presented grade 2/6 lameness of the operated limb at the 10-day follow-up, stifle palpation or manipulation did not evidence pain.

Radiographic follow-up after 30, 60, and 90 days confirmed the adequate position of both the patella and trochlear prosthesis, maintenance of the femoral–tibia–patellar axis, and no signs of osteomyelitis and/or osteopenia at the region of the base plate. The cat presented with a 1/6 grade lameness in the operated limb at the 30-day follow-up and had apparently normal mobility and activity 60 days after surgery. After 2 years, the cat presented normal walking without lameness, and regular movement speed and range of motion in the operated stifle.

Discussion

The placement of a trochlear prosthesis is a relatively novel technique to adequately restore patellar function in dogs affected by severe patellofemoral arthritis, cartilage loss, and chronic joint pain.[11] Although the successful application of a trochlear prosthesis to treat a cat has been recently reported, further studies on customized components for cats of different sizes are needed. Although Jaworski et al[7] also highlighted the increased risk of patellar tendon thickening, this complication was not observed in this case at the 2-year follow-up.

Trochlear ridge augmentation with ultra-high molecular weight polyethylene (UHMWPE) prostheses or placement of a trochlear ridge prosthesis (TRP) is recommended whether the hyaline cartilage of the articular surface is preserved; however, the use of TRP in cats has not been evaluated. Both TRP and UHMWPE can preserve the cartilage, reduce osteoarthritis progression, and provide a quick return to normal function.[12] [13] In the present case, the degenerative joint disease induced the formation of fibrocartilaginous tissue to replace worn cartilage.[3] Moreover, the height of the medial trochlear rim was insufficient to stabilize the patella[1]; thus, the combination of a shallow trochlear groove with a low medial trochlear rim and a degenerate trochlear articular surface made the prosthetic replacement of the whole trochlea appropriate.[7]

Dogs submitted to the placement of trochlear prostheses may present recurrent patellar dislocation, prosthesis loosening, infections, and patellar tendinopathies.[11] Since a trochlear prosthesis for dogs has high rims (reflecting the higher trochlear ridges in dogs compared to cats), its placement in cats is expected to be more than adequate to prevent patella redislocation, particularly in cats without varus or torsional deviation.[14] In this case, a trochlear prosthesis with adequate dimensions for the particular cat was used to ensure the stabilization of the patella, and no complications were observed.

In addition to the good healing capacity of a young cat, the perforations in the base plate stimulated the trochlear prosthesis osteointegration. Moreover, the Morse taper fixation system significantly contributes to preventing loosening between the two components of the prosthesis.[7] The lack of osteoarthritis progression and patellar thickening signs may be explained by the low friction coefficient of the diamond-like amorphous carbon-covered trochlear prosthesis surface.[3]

In this case, the release of the retinaculum and desmotomy of the medial patellofemoral ligament were able to neutralize the forces that previously caused medial luxation of the patella, and no additional techniques were needed. Capsulorrhaphy was initiated with simple sutures to assess the tension of the patella on the prosthesis. Flexion, extension, adduction, abduction, and twisting movements in all directions were performed to confirm adequate patellar positioning.[2] [14] At the 60-day follow-up, the patient showed normal walking without lameness, willingness to climb, and running without any restrictions. The postoperative radiographs revealed an adequate positioning of the prosthesis, resolution of patellar dislocation, and efficient restoration of the extensor mechanism.[6] [9]

Although the surgical planning of this case was based on radiographic evaluation, the use of computed tomography scanning would allow more precise measurement of volumes and angles of the medial and lateral trochlear crests, as well as diagnosing external torsions of the distal femur and trochlea.[7] Failure to correct the topography of the distal femur through femoral trochlear osteotomy may increase the risk of patellar redislocation.[1]

Studies with a sufficient number of patients are needed to validate this technique for cats, evaluate the biomechanical consequences, address indications and contraindications, and investigate the combination with adjuvant techniques such as tibial tuberosity transposition, soft tissue release/imbrication, and others.[1]

The placement of both the base plate and trochlear prosthesis was uncomplicated. In this case, even though the prosthesis was designed for the narrower canine patella, partial parasagittal patellectomy was not required since the prosthesis proved wide enough to adequately accommodate the patella within the prosthetic sulcus.[1] [2] Nevertheless, the development of customized trochlear prostheses for felines with wider prosthetic grooves and lower rims may avoid the need for adjuvant partial patellectomy in severe cases.

At the 2-year follow-up, the cat did not show lameness for daily physical activities and no analgesics and anti-inflammatories were further needed; thus, it can be assumed that the placement of a canine #3 trochlear prosthesis was effective. Even so, the development of specific prostheses for other sizes of cats and different degrees of joint damage is encouraged.

Conclusion

The use of a trochlear prosthesis to replace the femoral patellar groove in cats has the potential to correct the alignment of the extensor mechanism, improve patellar stability, and reduce patellofemoral osteoarthritis-induced lameness.

Conflict of Interest

None declared.

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Publication History

Received: 24 May 2024

Accepted: 14 February 2025

Article published online:
15 December 2025