The superficial digital flexor tendon originates on the lateral femoral head and lateral sesamoid. It forms both medial and lateral retinacular attachments at the tuber calcanei, coursing distally over the plantar aspect of metatarsals II to V before inserting on the middle phalanges. It forms the most superficial part of the Achilles tendon (Fossum, 2019). Superficial digital flexor tendon luxation is an uncommon condition in dogs, with Collie breeds and Shetland sheepdogs overrepresented (Bernard, 1977; Mauterer et al, 1993; Solanti et al, 2002). Luxation is typically caused by a rupture of the medial or lateral retinacular insertion on the calcaneal tuberosity, with the former, which leads to lateral luxation, being most common.
The clinical presentation of dogs with superficial digital flexor tendon luxation ranges from mild to severe lameness, and gait abnormalities can be intermittent (Slatter, 2003). Associated effusion is palpable at the tuber calcis on orthopaedic examination in most cases (Johnston and Tobias, 2018). Superficial digital flexor tendon luxation in acute cases can be achieved by manual manipulation of the tendon towards the medial or lateral direction. In chronic cases it can be more challenging to manipulate the tendon, because of it becoming adhered to soft tissue in the luxated location. Such cases present with swelling at the level of the tuber calcanei. Conservative management, with or without external coaptation, has been shown to result in chronic lameness and incomplete return to function (Mauterer et al, 1993), so surgical treatment is usually recommended.
Surgical treatment involves reduction of the tendon into the central groove, resection of excessive retinacular tissue and suturing the retinaculum (Fossum, 2019). Goh et al (2022) reported the success rate following surgical stabilisation as 73%.
Case presentation
A two-year-old, male, neutered, crossbreed dog, imported from Portugal was presented with a history of right pelvic limb lameness and intermittent skipping during walks. Gait abnormalities had been first noted 1 week after a collision with an electric fence, which was immediately followed by a period of intense exercise. Lameness and marked swelling of the right tarsal joint were noted 3 weeks after the incident. Occasional spontaneous vocalisation during walks was also reported. Medical management consisting of strict rest and analgesia (grapiprant 2 mg/kg once daily and paracetamol 12 mg/kg twice daily) was initiated for 4 weeks, but the clinical signs did not resolve. A computed tomography scan was performed and revealed marked soft tissue swelling around the distal calcaneal tendon extending around the os calcis. There was a fluid pocket extending proximally from the os calcis, between the distal gastrocnemius and superficial digital flexor tendon consistent with the distension of the calcaneal bursa. The tendon could not be traced to the medial aspect of the os calcis (Figure 1). The left tarsus showed no gross abnormalities apart from mild osteophytosis in the second metatarsophalangeal joint.
A 7-week period of strict exercise restriction and repeat of medical management was instituted and subsequent mild improvement was noted; however, lameness recurred once exercise was increased. The patient was reported as lame on the left pelvic limb 3 months later, with a similar palpable swelling in median calcaneal bursa region. Referral was sought at this stage.
Clinical examination
The patient was bright, alert and responsive. The patient's body weight was 33.5 kg with a body condition score of 5/9. The heart rate was 68 beats per minute, mucous membranes were pink and moist with a capillary refill time of under 2 seconds. Respiratory rate was 24 breaths per minute and the rectal temperature was 38.3°C. Orthopaedic examination revealed off-loading of the left pelvic limb at the stance, marked tarsal swelling and palpable lateral superficial digital flexor tendon luxation bilaterally. Dynamic gait assessment revealed a moderate pelvic limb lameness.
Diagnostic techniques
Ultrasound scans of the tarsus were performed. The right conjoined tendon had a normal diameter and normal fibre pattern. The superficial digital flexor tendon had a normal appearance and thickness proximally, but became thickened and irregular as it extended distally, with hypoechoic areas dissecting between the hyperechoic fibres (Figure 2). The course of the superficial digital flexor tendon was abnormal, and it could not be followed medially to the os calcis because it extended laterally. A mild increase in fluid was present in the calcaneal bursa. The right gastrocnemius was mildly heterogeneous distally.
The left conjoined tendon and gastrocnemius had no abnormalities detected. The left superficial digital flexor tendon showed a normal course during the ultrasound and was not manipulated out of this position during the scan. It became thickened and irregular at the distal aspect, with a similar appearance to the right superficial digital flexor tendon. There was a very large anechoic fluid-filled swelling of the calcaneal bursa (Figure 3).
Fine needle aspiration of the distended left calcaneal bursa was performed, and cytological analysis was consistent with a mild, mononuclear inflammation. There was no evidence of neutrophilic inflammation or infection and there were no atypical cells seen.
Problem list and differential diagnosis
The problems identified were bilateral pelvic limb lameness and swelling of the tarsal area with fluid accumulation in the calcaneal bursa.
Differential diagnoses included:
- Partial tear of the superficial digital flexor tendon
- Tendinopathy
- Bursitis.
The clinical examination confirmed a diagnosis of bilateral spontaneous lateral luxation of the superficial digital flexor tendon on flexion of the tarsus.
Surgery
Bilateral abrasion calcaneoplasty and primary retinaculoplasty was performed (Johnson and Davis, 2022). The patient was positioned in sternal recumbency with the limbs hanging caudally. After a routine sterile preparation, a caudal medial approach was performed via a curvilinear incision in the skin and fascia over the left tarsus. The fibrocartilaginous cap was located, and the adhesions were excised to access the tuber calcanei. The calcaneal bursa was sharply incised parallel to the tendon. The groove of the calcaneus between the medial and lateral processes of the tuber calcanei was subjectively flat (Figure 4). The remaining part of the calcaneal tendon was identified and protected from iatrogenic injury and the groove was deepened with a number 2 burr to a depth of 2 mm.
The excessive retinaculum was excised, and the remaining tissue was closed using non-absorbable 2-0 nylon suture (Monosof, Medtronic) in a simple continuous pattern. The fascia was closed with a vest-over-pant suture technique, and the skin was closed in a simple continuous pattern using 3-0 monofilament suture (Monocryl, Ethicon). The same procedure was repeated on the right pelvic limb. Post-operatively, the superficial digital flexor tendons were stable through full range of motion and could not be manually luxated.
Postoperatively, a custom-made fibreglass half cast in a modified Robert Jones dressing was placed on each pelvic limb. The dressings were changed every 3 days for the first 10 days, and every week thereafter. Whilst the dressings were intended to be maintained for 4 weeks, they were ultimately removed at 3 weeks postoperatively to prevent deterioration of mild pressure sores. Strict crate rest was advised instead for a further 2 weeks. Analgesia consisting of grapiprant 2.8 mg/kg per os once daily and paracetamol 20 mg/kg per os twice daily was prescribed for 5 days after the surgery.
Outcome and follow up
The patient was re-examined 4 weeks after the surgery. At this time, management consisted of strict crate rest with lead walks for toilet purposes only. Dynamic gait assessment was unremarkable. The surgical sites were fully healed, and the superficial digital flexor tendons were stable through full range of motion and could not be luxated manually. Minor soft tissue wounds were present on the left lateral tarsus consistent with healing pressure sores. The recovery plan involved gradual return to exercise, starting with 5 minutes lead walks 3 times daily and increasing these by 5 minutes intervals every week. The owner reported recurrence of lameness 3 weeks later (7 weeks after the surgery), which was associated with off-lead activity and running. After a further week of resting period and return to advised recovery plan, the lameness did not recur.
The patient was re-examined 9 weeks after surgery, with no lameness noted on gait examination. A new partial thickness wound over the left lateral malleolus was noted (suspected incidental) and deep infection was discussed as a possible source of previous lameness. However, this was thought unlikely as the incidence coincided with the re-introduction of off-lead activity and did not continue after recommencing the advised exercise schedule. Both superficial digital flexor tendons were stable on manipulation and there was no pain elicited. A telephone follow up was carried out 13 weeks post-surgery, with no lameness was reported by the owner at this stage.
Discussion
This case report presents a successful treatment of bilateral superficial digital flexor tendon luxation using the abrasion calcaneoplasty and primary retinaculum repair method described by Johnson and Davis (2022). Lateral luxation of the superficial digital flexor tendon, as seen in this case, is more commonly described in the literature than medial (DeCamp et al, 2016). The aetiology of lateral luxation appears to be tearing or redundancy of the medial retinacular attachment; however, the cause of this remains unknown (Solanti et al, 2002). Trauma is a possible factor – in the present case, the reported collision with an electric fence was suspected to have either caused or contributed to the pathology noted. However, because the clinical signs were initially subtle and not noted until a week after the collision, this may have been incidental. It has been postulated that one of the predisposing factors in superficial digital flexor tendon luxation is dysplasia of the tuber calcanei, with the groove being shallow or absent (Reinke et al, 1993).
The surgical method used in the present case has been described in the literature (Johnson and Davis, 2022). This technique is proposed to address the speculated deformity of a shallow calcaneal groove. This technique was easily performed through the standard incision, but does require caution to prevent iatrogenic injury to the insertion of the common calcaneal tendon proximally. In this case, calcaneoplasty was elected and performed using subjective assessment of the gross appearance of the calcaneous at the time of surgery. In the future, planning with computed tomography imaging could be useful, should objective measurements be reported. Another technique described (Nam et al, 2022) was a block recession calcaneoplasty alongside suture anchors to repair the retinaculum and temporary Kirchner wire placement, with a successful outcome and no further superficial digital flexor tendon relaxation after removal of the restraining pin.
The decision to prescribe grapiprant was based on this drug having previously been prescribed by the referring practice for the same complaint, with no noted adverse gastrointestinal adverse effects despite a history of previous occasional gastrointestinal problems secondary to meloxicam administration in this patient. Grapiprant is typically indicated for treatment of osteoarthritic pain and inflammation rather than pain associated with surgical trauma, and a different choice of non-steroidal anti-inflammatory drug may have been more appropriate in this patient.
Postoperative care involved placing external coaptation with the aim to maintain this for a period of 4 weeks. Immobilisation following orthopaedic conditions is reported to have a high risk of soft tissue complications (Meeson et al, 2011). In the present case, mild complications involving erythema and pressure sores were first present at 23 days post-surgery and so the splints were removed, leading to a successful outcome. The necessity of external coaptation, and the form this should take, following surgical stabilisation of superficial digital flexor tendon, is not well-defined. In one study (Goh et al, 2022), surgical failure was more common in cases managed without rigid immobilisation (consisting of calcaneotibial screws, splints, bi-valve casts and external skeletal fixators), although this result was not statistically significant. Goh et al (2022) found that immobilisation for a period of longer than 6 weeks was not associated with reduced risk of surgical failure and the majority of complications were directly associated with the presence of dressings. External coaptation following superficial digital flexor tendon luxation repair was placed for a maximum of 12 days in one study, with good results (Jury, 2021). Using orthotics as a coaptation method postoperatively may also be considered and may reduce the incidence of dressing-related complications (as seen in this case) in addition to providing early support of the repair (Kaufmann and Mich, 2013). They have the advantage of being easily removed and replaced but do typically involve a greater initial cost and need for measurement collection. Further work to determine the optimal period of external support in superficial digital flexor tendon stabilisation is required. Reports of the long-term outcome of this technique are currently limited and the long-term impact of cartilage removal is unknown; this should be bourne in mind when selecting this procedure. Adjunctive physiotherapy can also be considered during the recovery to help with the balance and strength restoration (Baltzer, 2020) – in this case consultation with a veterinary physiotherapist was offered but was declined by the owners.
Conclusions
This case presents the successful surgical management and short-term follow-up in a dog with the uncommon diagnosis of bilateral superficial digital flexor tendon luxation. The luxation may have been associated with mild electrocution or vigorous exercise, though this was not definitively determined and the underlying cause of this condition is not fully understood. Surgical treatment consisted of bilateral retinaculoplasty and abrasion calcaneoplasty. Following the surgery, external coaptation was used for 3 weeks in addition to exercise restriction. Mild soft tissue complications were associated with splinted dressing application but did not require additional treatment. Follow up regarding long-term outcome of this technnique is needed.
KEY POINTS
- Bilateral superficial digital flexor tendon luxation is uncommon and surgical stabilisation is advised.
- Superficial digital flexor tendon luxation can be associated with a previous history of vigorous exercise and trauma.
- Surgery, consisting of abrasion calcaneoplasty and retinaculum repair, can be considered as a primary treatment method.