Arthroscopic Assessment for Patellar Injuries and Novel Suture Repair of Patellar Fracture in a CatS

Introduction

Patellar injury, including luxation and fracture, is uncommon in cats.^1–5 Acquired patellar luxations, as opposed to those of a congenital origin, are the result of either direct or indirect trauma.² Similarly, fracture of the patella may be the result of either direct or indirect mechanisms. Indirect mechanisms, occurring when sufficient force causes failure of the extensor mechanism, frequently result in transverse fractures of the patella, while direct mechanisms are more likely to result in comminuted and open fractures.⁶ With the majority of feline patellar fractures being stress fractures that are transverse in configuration, only 20% of reported cases are a result of direct trauma.^1,4

Diagnosis of patellar luxation is initially achieved by joint palpation and evaluation of patellar stability, and confirmation and characterization of patellar fracture is confirmed by radiographic analysis.⁷ Goals of treatment of patellar injuries are to restore the functional extensor mechanism, articular congruity, and full range of motion.⁶ Luxations of the patella may be managed either conservatively or treated with surgical repair, depending on the grade of luxation and presence of clinical signs.^7,8 Treatment options for patellar fracture in cats have included conservative management and surgical repair, usually with the use of a circumferential wire plus figure of eight wire configuration.^1,3,4,9 When surgical repair is not an option, partial patellectomy has also been performed.^1,3,4 Arthroscopic surgery of the stifle, noted to have the potential for decreased recovery times and reduced postoperative morbidity when compared with open surgical repair, has been used with success for partial patellectomy in the small animal patient.¹⁰

Case Report

A 4 yr old indoor/outdoor spayed female domestic shorthair weighing 4.5 kg presented to the primary care veterinarian for an acute nonweight-bearing left hind limb lameness. Physical examination findings were within normal limits with the exception of the left hind limb. Pain on palpation of the stifle was detected, but no other specific findings were noted. Due to the cat’s fractious nature and soft-tissue swelling around the stifle, direct palpation of the stifle and patella was unrewarding. Radiographs of the stifle taken at that time revealed a possible patellar fracture. Due to obliquity of the radiographic views, it was difficult to confirm either fracture configuration or patellar location. The owners elected conservative management at that time, and a soft padded bandage was applied with instructions to keep the patient confined. No improvement was seen over the next 2 wk, and the patient was referred for further evaluation and treatment.

On presentation, the soft padded bandage was removed and the cat showed a nonweight-bearing left hind limb lameness. Pain response was elicited with any palpation and manipulation of the stifle such that a thorough examination without sedation was not possible. The stifle was palpably enlarged with suspected periarticular fibrosis. Radiographs were obtained that revealed a lateral longitudinal patellar fracture as well as a medial patellar luxation (Figures 1A, B). The cat was admitted for arthroscopic examination of the stifle and repair of the patellar fracture. Preoperative complete blood count^a and serum biochemical analysis^b,c were within normal limits.

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The cat was premedicated with 0.1 mg/kg hydromorphone^d, 6 mg/kg ketamine^e, and 0.1 mg/kg midazolam^f via intramuscular injection. General anesthesia was induced with propofol^g given to effect and maintained with isoflurane^h via an anesthetic vaporizer. An epidural was administered with 0.1 mg/kg morphineⁱ with 0.9% saline for volume. Cefazolin^j (22 mg/kg) was administered IV q 90 min during anesthesia. Palpation of the stifle under anesthesia revealed a grade 4 medial patellar luxation.

Medial and lateral parapatellar tendon stab incisions were made. The two ports were made at the proximal aspect of the tibia; the medial port was made immediately medial to the distal patellar tendon; the lateral port was made at a similar level of the distal patellar tendon and lateral to the patellar fragment. A 1.9 mm rigid 30° arthroscope^k was inserted into the lateral port. An arthroscopic l-probe was inserted into the medial port to manipulate tissues to increase visibility. The cranial and caudal cruciate ligaments were intact. The menisci were evaluated, as much as was possible without either damaging or otherwise compromising the integrity of the cranial cruciate, and were found to be intact. The patella was observed to be in the medial pouch of the stifle (Figure 2A). The patella could not be replaced into the trochlear groove. The distolateral aspect of the patella revealed an absence of cartilage and exposed subchondral bone (Figure 2B). The area over the trochlear groove and lateral compartment were examined; however, the smaller patellar fragment could not be observed due to abundant fibrous tissue.

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A medial parapatellar skin incision was made that extended from just proximal to the patella to the proximal aspect of the medial tibia. Once the skin and subcutaneous tissues were incised and elevated from the deep fascial tissue, the patellar tendon, patella, and patellar fragment could be identified. At this time it was noted that the patellar tendon had a proximal longitudinal tear such that the proximal patellar tendon was attached to both the patella and the lateral patellar fragment. Approximately 20% of the patellar tendon appeared to be attached to the smaller patellar fragment. As the lateral femoropatellar ligament was intact, the patella and patellar fragment were exposed by incising through the fibrous tissue located between the patella and the patellar fragment. A medial soft tissue release was performed to reduce the main patellar body into the trochlear groove. The injured quadriceps tendon proximal to the patella was apposed with 0 polydioxanone^l in a horizontal mattress pattern. A second horizontal mattress suture was placed that encircled the patella and lateral patellar fragment and held them in apparent reduction. These sutures effectively held the repaired patella in the trochlear groove. Following reduction, manual luxation of the patella was not possible. A lateral patellofabellar suture, with #40 monofilament nylon^m was placed to act as an internal splint and relieve tension on the primary fracture repair. Care was taken during placement of the nylon suture to prevent intra-articular placement. The suture was tightened such that it prevented medial luxation. Prior to closure, the arthroscope was reinserted to assess patellar location and fracture reduction (Figure 2C). Closure was routine with 3-0 poliglecaprone 25ⁿ suture used for the subcutaneous and intradermal skin closures.

Anesthetic recovery was unremarkable. Postoperative pain management was maintained with 0.2 mg hydromorphone given IV pro re nata for the first 24 hr postoperatively. A single dose of 0.1 mg/kg meloxicam^o was given subcutaneously, and 0.15 mg/kg buprenorphine^p per os q 8 hr. The cat was discharged on the second postoperative day. At that time, the cat was using the limb with a toe-touching lameness.

The owner was contacted 5 wk following surgery and reported that the cat was doing very well. She was using the limb normally, although she was still being kept inside and her activity was being restricted. Follow-up examination and radiographs were taken 9 wk postoperatively. At that time, the owner reported no lameness, and the cat was still being kept inside. Examination revealed normal, pain-free range of motion of the stifle, no palpable crepitus, and the patella was unable to be luxated. Radiographs confirmed normal location of the patella; the fracture gap between the patella and fragment remained with minimal callus formation (Figures 3A, B).

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Discussion

The patella, a sesamoid bone in the extensor mechanism between the quadriceps tendon and the patellar tendon, acts as a lever for stifle extension, thereby increasing the force of the quadriceps muscle. Due to its location, the patella is prone to high tension and compressive forces at the stifle joint that have implications both for method of patellar injury as well as for treatment and outcome.⁶ As removal of the patella leads to degenerative joint disease, every effort should be made to preserve the patella and restore the function of the stifle joint.^4,10 Owing to their natural behavior, which includes high level jumping, cats are particularly sensitive to alterations in the extensor mechanism of the stifle, and surgical repair of patellar fracture should be attempted whenever possible.⁴

Patellar injuries are very uncommon in cats.^2–5 Luxations are more common than fractures and may be either congenital or acquired.^2,5,8 Luxations are typically medial, as in this case.^2,5,8 The cat in this case presented with a presumed acquired luxation, and in these cases, the etiology is most commonly trauma.² However, the cat in this case presented with a patellar fracture as well, and feline patellar fractures are typically caused by indirect trauma, such as stress fractures, which are usually transverse.^2–4 Interestingly, in this case, the cause was presumably traumatic, and the fracture was longitudinal. Fractures due to trauma represent only 20% of feline patellar fractures and may be accompanied by patellar luxation.⁴ The lateral longitudinal patellar fracture and associated luxation of the feline stifle in the presenting case represents an uncommon injury in cats.^3,4

Patellar injury is routinely suspected based on history and clinical signs.² Depending on the grade of patellar luxation, lameness may or may not be present, whereas acute traumatic fracture of the patella is more likely to case acute clinical signs of lameness.⁵ Patellar injury is confirmed by palpation of the joint, evaluation of patellar stability, and radiography.² Grading of patellar luxation in cats is the same as that used to describe the severity of patellar luxation in dogs.^7,8 Grade 1 luxations can be manually luxated, but return to normal position once digital pressure is removed. In grade 2 luxations, the patella can be displaced by digital pressure or rotation of the tibia, with the patella being able to be reduced manually and remaining in place until manipulation. Grade 3 luxations are present at the time of examination and, although able to be reduced manually, they will reluxate immediately. Grade 4 luxations are luxated at presentation and cannot be reduced. Grade 1 luxations in cats rarely produce clinical signs.^7,8 Grade 2 and 3 luxations are variable in their degree of severity of clinical signs, whereas grade 4 luxations will produce pain, alterations in gait, and potential locking of limb in extension.^7,8 On routine ventrodorsal and lateral radiographs, a luxated patella will be displaced from the trochlear groove. Skyline radiographs with the joint in full flexion are used to assess depth of the trochlear groove, joint abnormalities, and to identify displaced fragments in patellar fragment.^2,3,7 Grade 1 and 2 luxations rarely demonstrate radiographic changes, whereas grade 3 and 4 luxations may.⁷ Although a stifle injury is typically obvious from physical examination, it can be difficult to accurately determine the exact ligaments involved in the injury based only on physical exam and radiographs. Surgical exploration and evaluation of the joint and ligaments is usually needed to confirm the extent of injury.⁷

Treatment choices for patellar injuries are either conservative or surgical in nature. Surgery is generally not recommended for patellar luxations in cats without clinical signs.^7,8 Grade 1 patellar luxations rarely require surgery, and grade 2 luxations may require therapy depending on clinical signs. Grade 3 and 4 luxations will typically require surgical intervention.⁷ A combination of bone and soft tissue reconstructive surgical procedures is needed to restore normal anatomy and improve patellar function.^4,7 For patellar luxation, various surgical methods have been described, including capsulorrhaphy, trochleoplasty, patelloplasty, wedge osteotomy, and transposition of the tibial tuberosity.^2,7 Sulcoplasty in adult patients, tibial tuberosity transpositions, or a combination of those two procedures, are thought by some to be the preferred surgical repair for patellar luxation in cats.² Reconstruction of damaged soft tissues is also crucial for repair of patellar function. Prognosis for surgically treated patellar luxation in the cat is good.^2,5,8

Conservative management of patellar fractures in cats may give satisfactory functional results although nonunion of the fracture frequently results.⁴ In contrast to traumatic injury, stress fractures are more likely to have intact soft tissues, thereby either preventing or minimizing displacement of fragments and allowing for greater success with conservative treatment.⁹ More commonly, surgical reduction of the fracture is performed.^4,6,9 Surgical repair of the fracture, which is typically transverse in cats, is usually performed with the use of interfragmentary Kirchsner wires, with or without the use of a tension band, cerclage wires, or small surgical screws.^3,4,9 Typical outcomes following surgical repair of patellar fractures in most species is union of the fracture and return of function in proportion to the severity of the insulting injury.⁶ Partial patellectomy may also be performed without complication in patients where the patella is considered nonreconstructable.^1,3,4 Every effort is made to conserve as much of the patella as possible, due to its significant importance in the extensor mechanism of the stifle.^4,6 Complete patellectomy is considered a salvage procedure when all other options have been exhausted and is considered particularly contraindicated in cats, due to their propensity for high elevation jumping, an action which requires an intact extensor mechanism.^4,9

Deterioration of knee function has been noted in human patients where internal fixation has been performed but only imperfect reduction achieved.¹⁰ To achieve perfect alignment of fracture pieces, an open surgical approach to the stifle is commonly needed. The drawback of that open approach is invasiveness and associated postoperative morbidity.¹⁰ Arthroscopy has been suggested to reduce the rehabilitation time and the postoperative morbidity in small animal patients with stifle injury.¹⁰ The use of arthroscopy has recently been reported for partial patellectomy in a canine patient, with significant reduction in post operative morbidity when compared with a more standard parapatellar arthrotomy.¹⁰ The clinical improvement seen following the use of arthroscopy combined with a modified surgical approach in the cat described in this report is consistent with the decreased discomfort seen in the reported canine patient (and in human patients) compared with open stifle surgery.¹⁰ To the authors’ knowledge, there are no reports in the veterinary literature of the use of arthroscopy in evaluation of patellar injury in cats. Diagnostic arthroscopy provided a thorough assessment of the patellar injury in this case (see supplementary Video I), which allowed for a more conservative surgical approach. Arthroscopy may be a useful adjunctive tool for assessment of traumatic patellar injury in cats.

In this patient, the decision was made to avoid a traditional arthrotomy to preserve as much as possible of the intact stifle anatomy. Due to the traumatic nature of the patellar luxation in the presenting case and lack of congenital malformations, a conservative surgical repair was performed, by way of a medial release, to allow the patella to return to its normal position in the trochlear groove. In contrast to the technique performed in this case, where exposure of the patellar fragments was gained by incising through fibrous tissue between the fragments, leaving the femoropatellar ligament intact, a parapatellar arthrotomy is generally used for repair of severe patellar luxations.⁷ The use of arthroscopy to correctly assess the patellar and soft-tissue injury in this case allowed the use of this more conservative and less invasive surgical approach. Typical surgical technique for medial patellar luxation is a lateral arthrotomy followed by lateral imbrications. The use of arthroscopy revealed an alternate option to approach the injury via an interfragmentary approach. This approach maintained the normal lateral support structures of the patella (joint capsule and patellofemoral ligament). The status of those structures could have been assumed to be intact prior to surgical intervention; however, arthroscopic confirmation allowed an alternate approach to be attempted with greater confidence.

In cats, repair of patellar fractures is usually performed by way of internal fixation with pins and/or wires.^4,9 Complications, such as wire pull out or breakage, further fracture of the patella, and fracture distraction have been noted with this type of repair.⁹ Though reported by some to be biomechanically inferior to pin and wire repair, circumferential wire may provide fragment reduction and soft-tissue reconstruction without causing further trauma to the patella.⁹ In this case, horizontal mattress sutures were used as a novel method of patellar fracture reduction. Fracture reduction with suture offered the theoretical advantages of easier application, greater resistance to cyclic failure, and perceived greater comfort for the patient compared with cerclage wire. Suture was also used due to the small size of the patient and relatively thin soft tissues such that complete avoidance of the joint could not be assured. The combined effect of the placement of two mattress sutures and the medial release was reduction of the fracture and prevention of luxation of the patella. Reduction of the fracture and return of the patella into the trochlear groove was confirmed on postoperative radiographs. Postoperatively the patient’s lameness had improved within 48 hr, and follow-up of the patient suggested that stifle function had been restored.

At the 5 wk postoperative mark, the cat was considered by the owners to be using the limb normally, though activity was still being restricted. Fracture healing assessed at 9 wk was less apparent on the radiographs than had been expected, although 9 wk postoperatively is too early to determine whether a fibrous nonunion is present (Figures 3A, B). Due to the high motion nature of the location of the injury, a delayed union would not be unexpected. At the most recent postoperative check of the cat presented in this case, clinical improvement indicated a return of function of the stifle joint, and no further intervention was deemed appropriate. The owners were given instructions to continue to allow a progressive increase in activity as part of the cat’s normal recovery and rehabilitation. The postoperative improvement seen in this cat is consistent with that seen in the few other documented cases of surgical repair of patellar fractures in cats, although the technique for repair differed. In transverse patellar fractures, pins and/or wires have been used for surgical repair, whereas partial edge patellectomy has been used in the few reported cases of longitudinal fractures.³

Conclusion

The clinical findings in this case suggest that arthroscopy may be a useful tool in the management of acute traumatic patellar luxation in the feline patient and that suture reduction of longitudinal patellar fractures, without use of standard arthrotomy, may be a viable means of patellar fracture repair in the cat.