Step ostectomy as a treatment for varus deformity of a metatarsophalangeal joint in a 4.5-month-old colt

Abstract

A 4.5-month-old quarter horse colt was presented with severe right hind metatarsophalangeal varus, present since birth. On radiographs, it was determined that there was a 16° deviation with the pivot point at the distal physis of the 3rd metatarsal bone. A step ostectomy in the sagittal plane corrected the deformity.

A 4.5-month-old quarter horse colt weighing 225 kg was admitted to the Veterinary Teaching Hospital at the Western College of Veterinary Medicine for an angular deformity of his right hind limb. His owners reported that the angular deformity had been evident since birth and that despite corrective hoof trimming since he was 2.5 mo of age, the deformity had not resolved. Lameness was never noted. The owners planned to use the colt as a barrel-racing prospect in the future.

Physical examination revealed that the colt was large for his age and systemically normal. Upon evaluation of his musculoskeletal system, he had mild tarsal valgus and severe metatarsophalangeal varus of his right hind limb. The flexor tendons were displaced medially at the metatarsophalangeal joint because of the abnormal conformation. Lameness was not evident and palpation and manipulation of the limb did not elicit pain.

Radiographs were taken to determine the angle of deviation and the pivot point and to evaluate the joints for degenerative joint disease. The distal physis of the right hind 3rd metatarsal bone showed mottling and an irregular lucency. There was flaring along the medial aspect of the physis. The same changes were present in the proximal physis of the proximal (1st) phalanx. There was no evidence of degenerative joint disease. A clear cellulose acetate sheet was placed over a radiograph of the dorsoplantar view of the right 3rd metatarsal bone and 1st phalanx. Two longitudinal lines were drawn on the acetate sheet, the 1st parallel to and through the center of the metatarsal bone and the 2nd parallel to and through the middle of the 1st phalanx. The point at which the 2 lines intersect is known as the pivot point. In this case, the pivot point was 1 cm proximal to the distal physis of the 3rd metatarsal bone and the angle of deviation was 16° (Figure 1). Radiographs of the left hind metatarsophalangeal joint were taken for comparison and were without visible bony lesions.

Figure 1.

Dorsoplantar view of the right metatarsophalangeal joint with geometric analysis. The degree of rotation is 16º and the pivot point is located 1 cm proximal to the physis. The horizontally orientated hatched area represents the dimensions of the proposed ostectomized bone.

A step ostectomy in the sagittal plane was recommended. The presurgical packed cell volume (PCV) and total protein (TP) were 0.38 L/L (reference range, 0.32 to 0.52 L/L) and 64 g/L (reference range, 59 to 73 g/L), respectively.

The colt was premedicated with xylazine (Anased; Novopharm Animal Health, Toronto, Ontario), 1.0 mg/kg body weight (BW), IV. Anesthesia was induced with ketamine (Vetalar; Bioniche Animal Health Canada, Belleville, Ontario), 2.0 mg/kg BW, IV, and diazepam (Diazepam; Sabex, Boucherville, Quebec), 0.1 mg/kg BW, IV, and maintained with inhalational 2% isoflurane (Isoflurane; Abbott Laboratories, Saint-Laurent, Quebec). Butorphanol (Torbugesic; Wyeth Canada, St. Laurent, Quebec), 0.2 mg/kg BW, IV, and cefazolin (Novopharm, Toronto, Ontario), 20 mg/kg BW, IV, were also administered and intraoperatively.

An incision was made through the skin, subcutaneous tissue, lateral digital extensor tendon, and the periosteum, which were then reflected to expose the distal portion of the right metatarsal bone. On the dorsal aspect of the metatarsal bone, a hole was drilled 1 cm proximal to the physis (at the pivot point) in the center of the bone. A 2nd hole was drilled 4.5 cm proximal to the 1st hole and an oscillating saw was used to cut through the bone and connect the 2 points. Using a pre-made sterilized aluminum template with the angle of the wedge to be removed from the bone, a 2nd cut was made proximad from the distal hole following the side of the template, toward the lateral side of the bone. Two more cuts were made parallel to the physis, from the proximal hole to the lateral side of the bone, and from the distal hole to the medial side of the bone. This completed the Z-plasty. The bone wedge was removed, and the 2 bone fragments were adjusted to correct the deformity. The bone wedge that was removed was inserted at the gap at the distal aspect of the ostectomy site as a cortical bone graft. The cut ends of the bone were apposed together with an 8-hole, 4.5-mm broad dynamic compression plate (DCP) applied to the medial side of the bone. One 5.5-mm cortex screw was inserted through the distal hole of the bone plate into the distal portion of the metaphysis. Intraoperative radiographs were taken to ensure that there was adequate reduction of the bone fragments. Three more 5.5-mm cortex screws were inserted in lag fashion proximal to the 1st screw and 3 4.5-mm cortex screws were applied in neutral position into the more proximal aspect of the bone plate. The most proximal screw was a 5.5 mm cortical screw (Figure 2). The periosteum and the soft tissues were closed by using a simple continuous suture pattern with 1-0 monocryl (Ethicon, Somerville, New Jersey, USA) and 2-0 monocryl (Ethicon), respectively. The skin was closed with a cruciate suture pattern using 2-0 Novafil (Syneture, Norwalk, Connecticut, USA). A half-limb cast was applied to the leg after postoperative radiographs had been taken to assess alignment of the bone and positioning of the implants relative to the bone, adjacent joint, and ostectomy site. The colt recovered from anesthesia uneventfully.

Figure 2.

Postoperative dorsoplantar radiographs of the right metatarsophalangeal joint showing the step ostectomy site. An 8-hole dynamic compression plate is located on the medial side of the bone. The bone wedge removed was placed at the distal aspect of the ostectomy site as a cortical bone graft.

The day following the surgery the colt was slow to rise and reluctant to use his right hind leg. He was maintained on phenylbutazone (Vetoquinol, Lavaltrie, Quebec), 3.0 mg/kg BW, PO, q24h for 1 wk, and ranitidine (Nu-Pharm, Richmond Hill, Ontario), 6.6 mg/kg BW, PO, q6h for 14 d. The lameness improved during this time. The colt was discharged 7 d following the surgery on a restricted exercise regimen.

The colt returned 1 mo following the surgery for cast removal and follow-up radiographs. The owners reported that he had been lame on the right hind leg for 2 d. The cast felt warm over the fetlock and he had a fever of 40.0°C. The cast was removed; some purulent discharge appeared from the incision site where the distal lag screw had eroded through the soft tissues. Radiographs of the ostectomy site showed that the edges of the bone fragments had become smoother and more rounded. There was periosteal reaction around the ostectomy site, indicating that the site was healing. A splint was applied to the leg and the colt was sent home with instruction to the owner to restrict his exercise to a small paddock.

The colt was readmitted 4 mo following his initial surgery for removal of the bone plate. His exercise had been restricted and his leg bandaged daily for 2 mo after cast removal. Hyperextension of the right hind fetlock was noted, which was likely due to weakened plantar support from chronic casting and bandaging. On radiographic examination, the ostectomy fracture lines were no longer visible. Under general anesthesia (as described previously), the bone plate and screws were removed. There were no complications until 8 d following the surgery, when the colt developed a grade 4 lameness in his left hind leg. Radiographs showed that there was degenerative joint disease in the proximal interphalangeal joint. Lucency in the distal condyles of the 1st phalanx was suggestive of osteochondrosis, which was not radiographically apparent on the initial films. The joint was injected with corticosteroids and the colt was discharged. No follow-up information is available at this time.

Angular limb deformity refers to deviation of a limb in the frontal plane. Carpal valgus is the most common deformity seen (80% of cases); however, fetlock deformities account for 15% of referred cases (1). Angular limb deformities may be present at birth or may develop in the postnatal period. The causes of angular limb deformity are complex and multifactorial. Suggested etiologies include flaccidity of periarticular soft tissue structures at birth; perinatal soft tissue trauma; placentitis; premature birth resulting in incomplete ossification of cuboidal bones; twin birth; unbalanced nutrition; excessive growth, exercise, or external trauma resulting in crushing of the physis (2).

A variety of treatment options are available to treat angular limb disease, including conservative and surgical options. Conservative options, including corrective hoof trimming, splinting, and stall rest, are typically reserved for younger foals (1).

In long bones that still have growth potential, surgical options for correcting deformities focus on manipulating growth at the physis. Total long bone growth occurs by endochondral ossification at the physis. Therefore, by manipulating either the medial or the lateral side of the physis, the deformity of the bone can be corrected. By either accelerating growth of one side of the physis through periosteal transection and stripping, or by retarding growth of one side of the physis by temporary transphyseal bridging. Periosteal transection and stripping and temporary transphyseal bridging must be performed before closure of the physis for these surgeries to have a desirable outcome. In the metatarsophalangeal or metacarpophalangeal joints, the procedures must be done before 2 mo of age (3). This colt was 4.5 mo old, so these procedures were not an option.

After closure of the physis, angular limb disease can only be corrected with an osteotomy, ostectomy, or both. Closing wedge ostectomy was the first procedure described as a possible treatment (4,5). This surgery involves removing a horizontal wedge of bone at or near the pivot point. Step ostectomy was described later and became the preferred treatment (6). The advantages of a step ostectomy over a wedge ostectomy are maintenance of bone length and maximal contact of the bone fragments, which allows for greater stability following internal fixation (6). The step ostectomy may be performed in the frontal plane or the sagittal plane, depending on the location of the pivot point. If the pivot point is located close to the metatarsophalangeal joint, the ostectomy in the sagittal plane is preferred, because the plate can be placed more distally on the medial or lateral side of the bone without damaging any important anatomic structure (7). In this colt, the ostectomy was performed in the sagittal plane because of its close proximity to the fetlock joint. A study reported in 1994 looked at 12 horses receiving step ostectomies in 15 limbs (6). Twelve of the 15 limbs resulted in pain-free, useful limbs. However, only 1 ostectomy was performed on a deformity at the metatarsus and only 3 of the 12 horses went on to have an athletic career. To date, there are no published studies that evaluate the long-term prognosis and athletic function of horses receiving step ostectomies for correction of angular limb deformity of the metatarsophalangeal joint.

Step ostectomies are uncommon, as most foals with angular limb deformities are corrected at a young age or are euthanized for financial reasons. However, the step ostectomy is a good choice of surgery after closure of the physis. The prognosis for this colt having an athletic career is fair to good, depending on the outcome of the contralateral limb pastern arthritis.