Patellar ligament rupture in the cat: repair methods and patient outcomes in seven cases

Abstract

The medical records of cats receiving surgical treatment for unilateral patellar ligament rupture between 1999 and 2012 at 12 referral centres in the UK and Ireland were reviewed. Seven cases were identified: six were caused by trauma and one was iatrogenic, occurring as a complication following surgical stabilisation of a tibial fracture. All cases were treated by sutured anastomosis of the ruptured ligament, with six of the repairs protected by a circumpatellar and/or transpatellar loop of suture. The stifle was immobilised by transarticular external skeletal fixation in three cases. No cases required revision surgery. No complications were reported. Final evaluation, performed at a median time of 31 days, determined five patients to have returned to acceptable or good limb function; two cases were lost to follow-up. The data suggest that, in cats, the current surgical techniques extrapolated from their canine counterparts for repair of a completely or partially ruptured patellar ligament are successfully used and result in acceptable limb function.

Introduction

Rupture of the patellar ligament (RPL), the portion of the tendon of insertion of the quadriceps femoris muscle that lies between the patella and the tibial tuberosity, ¹ is an uncommon injury that requires repair to re-establish the extensor mechanism of the stifle joint. ²

In humans, injuries to the patellar ligament (PL) tend to be traumatic in the younger patient, and secondary to age-related degenerative change in the older patient, ³ although they have also been associated with systemic disease or medication. ⁴ RPL in veterinary species is commonly traumatic in nature, although spontaneous bilateral rupture associated with fluoroquinolone treatment has been reported in the dog. ⁵

Techniques described for the repair of RPL have largely been empirical, based on our understanding of tendon and ligament healing, and are well reported for the dog. The techniques involve primary ligament repair where possible, with additional sutures placed to protect the sutured anastomosis from the potentially disruptive tensile force associated with quadriceps muscle contraction.^6,7 Alternatively, or in addition, a transarticular external skeletal fixator (taESF) can be placed to immobilise the stifle joint at a near-normal standing angle.^5,8–11 The use of a fascia lata autograft, either to augment a primary ligament repair or to span a defect in the ligament in chronic cases of rupture, has also been described.^7,9 Most recently, reconstruction of the PL has been described using an ipsilateral PL graft augmented with a modified hamstring graft. ¹¹ Postoperatively, immobilisation of the stifle joint is recommended, either by external coaptation or placement of a taESF.^12,13 To our knowledge, case reports describing the surgical techniques and outcomes after repair of RPL in the cat have not been published.

To date, the largest published study of RPL in the dog reported eight cases treated over a period of 9 years, although the authors were unable to draw any definitive conclusions about the most appropriate method for repair. ² No large-scale studies have been conducted to report the treatment modalities and outcomes of PL repair in the cat, and this lack of data means that little information is available to inform the selection of optimal treatment methods or to advise on the anticipated outcome following treatment.

Here, we report a multicentre, retrospective evaluation of the surgical techniques used for the repair of RPL in cats in the UK and Ireland, with the aim of identifying factors in treatment selection that are associated with an optimal outcome.

Materials and methods

Case selection

The medical records of cats with RPL from 12 multidisciplinary referral centres in the UK and Ireland between 1999 and 2012 were reviewed. For inclusion in the study, each case was required to have had an attempted surgical repair of unilateral RPL with a documented surgical procedure and follow-up information available.

Review of the medical records

Age, breed, sex, affected limb, cause of injury, extent of damage to the ligament, method of identification of the injury and imaging features (if applicable), time between injury and surgery, surgical procedure and postoperative complications were documented. All available follow-up information, including limb use, was recorded. The cause of injury was classified as traumatic, iatrogenic or spontaneous. The extent of damage was classified as complete or partial rupture of the PL. Details of the surgical procedure required were suture pattern used to appose the ruptured ends of the ligament; method of protection of the sutured anastomosis; any additional augmentation of the repair; and the method of immobilisation of the stifle if used.

Minor complications were defined as those not requiring surgical intervention for resolution. Major complications were defined as those requiring further surgical intervention for resolution or resulting in an unacceptable outcome for the patient.

Outcome was classified as good if the patient returned to pre-injury levels of activity with no reported lameness or requirement for medication to achieve this level of function at the last evaluated time point. Acceptable function was defined as restoration to near-normal pre-injury levels of activity for the patient but where a mild lameness remained that was not considered to reduce the quality of life or require medication to achieve it. Poor function encompassed all other outcomes, including failure of surgical repair, muscle contracture, abnormal patella position, moderate or severe lameness, or euthanasia.

Results

Between 1999 and 2012, 5/12 referral hospitals identified seven cases of RPL that fulfilled the inclusion criteria.

All cases were domestic shorthairs. Median age at presentation was 2 years (range 1–12 years). Four patients were male (two neutered) and three were female (all neutered). Three cases of left and four cases of right RPL were identified.

Six cases of RPL were due to non-iatrogenic trauma, either known or presumed based on the findings of clinical examination. One case was iatrogenic, resulting from a complication of tibial fracture stabilisation.

At presentation, five cases were deemed to be acute injuries and two were chronic. Five of the ruptures were complete and two were partial.

As part of the clinical evaluation, radiography was carried out in six cases; in one case no information about imaging was recorded. Radiographic findings included patella alta in four cases; the patella position was not recorded in two cases. Other changes identified were stifle effusion (two cases) and tibial tuberosity fracture (one case).

Concurrent injuries were present in three patients, including patellar fracture (n = 1), fractures in the ipsilateral and contralateral tibiae (n = 1), and nasal and other trauma-related soft tissue injuries (n = 1). No concurrent medical conditions were identified at the time of presentation in any of the cases.

Median time from injury to repair was 6 days (range 3–50 days); in one case, this information was not available.

Sutured anastomosis for apposition of the ruptured ends of the PL was performed in all cases. Suture patterns used included the locking loop (n = 4) and a form of mattress suture (n = 2). Suture pattern was not recorded in one case. The suture types used for anastomosis were recorded as polydioxanone (n = 3), nylon (n = 1) and polypropylene (n = 1); suture type was not recorded in two cases. Suture calibre was recorded for four cases as 3.0 metric in three cases (one each of polydioxanone, nylon and polypropylene) and 3.5 metric in one case (polydioxanone).

Protection of the primary anastomosis was performed in six cases. This was achieved by the placement of loops of suture material in a circumpatellar (n = 4) or a transpatellar (n = 2) position proximally, and through a tibial bone tunnel distally. The protective suture loop was either monofilament nylon leader line (n = 3), stainless steel orthopaedic wire (n = 2) or polydioxanone (n = 1). Two repairs were augmented with a fascia lata graft.

Postoperative immobilisation to protect the surgical repair of the PL was provided by a taESF in three cases. In the remainder of the cases (n = 4), the stifle joint was not immobilised.

All patients were discharged from hospital. Median time from surgery to discharge was 3 days (range 1–7 days). Information from reassessment after discharge from hospital was available for five patients; no follow-up was available for two patients. Median time to final evaluation was 31 days (range 16–48 days). The five cases with documented follow-up had no recorded complications, and the occurrence of complications after discharge from hospital was unknown in the remaining two cases.

Median time from surgery to removal of taESF was 43 days (range 31–45 days).

Five cases (71%) were reported to have returned to acceptable limb function at final evaluation; in two cases (29%) limb function at final evaluation was unknown or the case was lost to follow-up.

Discussion

PL injury is a relatively uncommon condition in companion animals and, although there are several isolated case reports detailing specific aspects of treatment,^6–11 only one case series has been published to date. ² However, to our knowledge, there are no published case reports or case series that describe the surgical repair of RPL and associated outcomes for the cat.

Proposed methods of surgical repair for RPL in dogs include a primary sutured anastomosis and protection of the anastomosis with a circum- or transpatellar suture with monofilament wire or nylon,^2,6,7 reconstruction with a PL graft, ¹¹ the use of a fascia lata or hamstring graft in the repair,^7,9,11 and postoperative immobilisation of the stifle with a taESF.^5,8–11 In the absence of data to suggest that they are inappropriate, the methods currently employed in cats are based on the reported treatment options used in dogs.

The present study was conducted to describe and evaluate the surgical techniques currently employed in the UK and Ireland for treatment of RPL in the cat, and attempt to draw conclusions about the most appropriate surgical treatment for this condition. Although this is the largest study of this injury to date, only seven cases were identified, precluding any meaningful statistical analyses. However, these cases have been described with the aim of providing some general observations and guidelines for treatment and establishing the prognosis for return to athletic function.

Predictably, trauma was identified as the most common (86%) cause of RPL and 71% of the cases presented as acute injuries. One case occurred as a direct result of complications from stabilisation of a tibial fracture. In this case, a tibial intramedullary pin had migrated proximally and lacerated the PL. Intramedullary pin fixation of tibial fractures is well described.^14,15 A higher incidence of trauma to the stifle joint has been demonstrated in dogs with retrograde intramedullary pin placement compared with normograde pin placement, ¹⁶ and a study in cats showed penetration of the PL in all cases of retrograde intramedullary pin placement in the tibia. ¹⁷ The single case of iatrogenic RPL in this study supports both the necessity for accurate tibial intramedullary pin placement and the recommendation for normograde pin placement to treat feline tibial fractures.

It is generally accepted that the ends of a ruptured tendon or ligament should be well apposed to allow optimum healing. The recommendation is to repair a RPL with a tendon suture pattern. ¹⁸ Studies in humans show that the strength of flexor tendon repair is roughly proportional to the number of suture strands crossing the repair site. ¹⁹ Canine studies have also shown a similar resistance to gap formation in suture patterns that have a higher number of strands crossing Achilles tendon repair sites. ²⁰ However, Strickland also notes that suture patterns with more strands crossing the repair site are likely to be technically more difficult and therefore carry a higher risk of iatrogenic tendon damage, potentially compromising the ability of the tendon to heal. ¹⁹ The use of locking suture patterns and larger calibre sutures has also been shown to increase tendon repair strength.^21,22 Gaps of more than 3 mm demonstrate a slower rate of healing and decreased ultimate strength 6 weeks after repair compared with no gap or gaps of <3 mm.^23,24 All the cases reported in the present study received a sutured anastomosis, suggesting that the recommendation for primary repair is widely accepted.

Protection of the sutured repair was recommended by Brunnberg et al, ² who used a temporary transpatellar or circumpatellar wire. Fatigue failure of wire sutures in areas of high motion is expected and is frequently reported,^2,8,9 and the use of nylon leader line has been proposed as an alternative owing to its superior resistance to cyclic fatigue.^6,7 In the present study, protection of the sutured anastomosis was performed in six cases with either wire, nylon or polydioxanone. No complications were associated with any of these, and although firm conclusions cannot be drawn from this relatively limited number of cases, an assumption might be made that the choice of suture material for the circum- or transpatellar loop is possibly of little consequence, although there is scope for future study to determine the superiority of any particular material for use in feline patients for this purpose.

The placement of a cast has previously been reported to be ineffective as a method of immobilising the stifle after repair of RPL in dogs. ¹⁰ The ability to effectively immobilise the stifle joint of veterinary patients by means of external coaptation is generally limited owing to anatomic constraints and it is often preferable to avoid the use of casts or dressings to immobilise the stifle joint in cats. The most common method of immobilisation of the stifle joint in this study was through the use of a taESF (n = 3, 43%). A study by Lister et al, ²⁵ who measured strain in the canine common calcanean tendon, found that the maximum strain in the Achilles tendon was not altered by the application of a taESF applied across the hock, but the minimum strain was reduced. This suggests that a taESF for joint immobilisation may exert its benefits by decreasing weightbearing and restricting activity as part of postoperative management. These conclusions may translate to immobilisation of the stifle joint, but future work is necessary to determine whether isometric quadriceps contraction across an immobilised stifle joint might significantly contribute to the distractive force acting on PL repair. Four of the cases in this study did not have any form of immobilisation, and follow-up is available for two of these. The lack of immobilisation did not appear to affect the development of complications or the eventual limb function as an acceptable outcome was recorded for all cases either with or without postoperative stifle immobilisation. This questions the necessity for surgical immobilisation of the stifle joint in cats, and it may be that postoperative confinement alone, for example to a cage, is sufficient to protect effectively a PL repair in feline patients.

Perhaps surprisingly, no postoperative complications were reported. This is may be owing to the small number of cases available to the study, coupled with the lack of follow-up in two of them. Complications likely to be encountered would be those commonly associated with any surgery similar in nature, including failure of repair requiring revision surgery, and morbidity associated with external coaptation/immobilisation. In a larger cohort of cases, a more realistic estimate of the incidence of major and minor complications might be demonstrable. Despite the apparent low incidence of such events, it would be prudent to consider the range of complications that might reasonably be anticipated, and to inform clients accordingly when faced with surgical treatment of RPL in the cat.

Although two cases were lost to follow-up, restoration of acceptable limb function was recorded in all cases in which the information was available. Thus, it would appear that cats have a good prognosis following surgical repair of a RPL. As with many retrospective studies of this nature, the data presented here highlight the need for a randomised, prospective study to determine the most effective method for the treatment of RPL in cats.

Conclusions

This retrospective multicentre study aimed to identify, through reporting the outcomes of the current surgical treatments used for repair of RPL in the UK and Ireland, the methods that may lead to optimal recovery and function. Being the largest study of this injury in cats to date, it highlights the uncommon nature of RPL and the lack of information surrounding treatment and outcomes. Nevertheless, the current surgical techniques employed in treatment of RPL appear to be successful in cats.