Morphology and correction of distal tibial valgus deformities

Introduction

Angular limb deformities are common in dogs (Johnson and others 1994) and often occur as a consequence of abnormal physeal growth secondary to partial or complete premature physeal closure (Carrig and others 1978, Carrig 1983). Premature physeal closure may result from trauma, chondrodystrophy or other musculoskeletal genetic diseases (Marretta and Schrader 1983, Sande and Bingel 1983). Deformities of the tibia represent 4·4 to 6·9 per cent of bone deformities in dogs and include valgus and varus deformities of the distal portion of the canine tibia (Ramadan and Vaughan 1979, Kasa and Kasa 1982, Marretta and Schrader 1983, Vaughan 1987, Johnson and others 1989, 1994, Fox and Machon 1992, Jevens and DeCamp 1993, McCarthy 1998). A varus angulation of the distal portion of the tibia, termed pes varus, has been described in dachshunds and is thought to be the result of metaphyseal dysplasia of the distal tibia with a presumed recessive mode of inheritance (Johnson and others 1989). Pes varus has been compared with human metaphyseal dysplasia (Pyle disease) (Jezyk 1985, Heselson and others 1979). Tarsal valgus could result from an abnormal growth of the lateral aspect of the distal tibial physis or from a disturbance of physeal growth (Hsu and others 1974, Vaughan 1987, Moon and others 1997). Abnormal growth of the distal aspect of the fibula and subsequent tethering and decreased lateral distal tibial growth has been suggested as a potential cause of valgus deformity of the distal portion of the tibia (Jevens and DeCamp 1993).

Little has been reported regarding the morphology, pathophysiology, specific treatment recommendations and long-term outcome in dogs with distal tibial deformities (Johnson and others 1989, Jevens and DeCamp 1993). In previous reports, the surgical correction of distal tibial deformities has involved the creation of a closing-wedge ostectomy and stabilisation of the bone segments with either internal or external skeletal fixation (Vaughan 1987, Johnson and others 1989, Fox and Machon 1992, Yanoff and others 1992, Jevens and DeCamp 1993, McCarthy 1998). Hinged circular external fixation (HCEF) has been used successfully for the correction of antebrachial deformities (Marcellin-Little and others 1998, Lewis and others 1999, Marcellin-Little 1999) and can be adapted to treat angulation and length deficits for distal tibial deformities. The purpose of this report was to assess the conformation and position of the tibia and fibula and to report on the results of surgical correction in a series of dogs with valgus deformities of the distal portion of the tibia.

Materials and methods

Results

The medical records of 16 dogs with 20 distal tibial valgus deformities, 12 unilateral and four bilateral deformities, were reviewed (Table 1). Rottweilers and Shetland sheepdogs treated at North Carolina State University during the study period were 12·2 and 12·3 times more likely to have such deformities than dogs of other breeds (95 per cent confidence intervals, 1·5 to 30·4 and 1·5 to 30·5, respectively). Age at presentation ranged from 5 to 12 months (median, 9 months). Thirteen of the 20 deformities were of the right and seven were of the left pelvic limbs. Valgus deformities ranged from 16 to 48° (median, 32°) in affected limbs and from 0 to 13° (median, 6°) in clinically normal limbs. Tibial length measured from radiographs ranged from 118 to 325 mm (median, 212 mm) for affected limbs of dogs with unilateral deformities and from 119 to 332 mm (median, 215 mm) for control limbs. Fibular length measured from radiographs ranged from 108 to 302 mm (median, 191 mm) for affected limbs of dogs with unilateral deformities and 102 to 303 mm (median, 201 mm) for control limbs. The tibial and fibular length deficits of affected limbs measured from radiographs ranged from −9 to 10 per cent (median, 1 per cent) and −9 to 17 per cent (median, 2 per cent). The ratio of fibular length to tibial length ranged from 0·82 to 0·96 (median, 0·89) in affected limbs and 0·86 to 1·0 (median, 0·92) in clinically normal limbs. The distal fibular area to distal tibial area ratio was 0·11 to 0·22 (median, of 0·18) in affected limbs and 0·10 to 0·23 (median, 0·14) in clinically normal limbs. Displacement of the head of the fibula from the proximal tibia ranged from 3 to 14 mm (median, 9 mm) in affected limbs and from 3 to 15 mm (median, 10 mm) in clinically normal limbs. The lateral malleolus was 8 to 21 mm (median, 12 mm) distal to the distal tibial articular surface in affected limbs and 9 to 19 mm (median, 12 mm) in normal limbs. The tibiotalar subluxation angle ranged from −2 to 12° (median, 4°) in affected limbs and from −5 to 3° (median, −2°) in clinically normal limbs.

Twelve unilateral and four bilateral valgus deformities of the distal tibial metaphysis were treated using conservative management (5/20), segmental fibular ostectomy (1/20), closing-wedge ostectomy with plate stabilisation (3/20), planar osteotomy corrected progressively using hinged circular external skeletal fixation (HCEF) (5/20) and dome osteotomy corrected acutely using HCEF (6/20) (1, 2). In the HCEF treatment groups, progressive and acute, the fixation consisted of a three-ring construct with a half-pin to further stabilise the single distal rings (Fig 3). These frames were constructed in accordance with previously recommended guidelines (Lewis and others 1998, 1999, Marcellin-Little 1999). In dogs with progressive angular correction, the adjustment phase ranged from 20 to 25 days. In the HCEF treatment groups, duration of hospitalisation ranged from 1 to 5 days (median, 1 day) for acute corrections and from 1 to 4 days (median, 1 day) for progressive corrections. Angular corrections ranged from 16 to 28° (median, 24°) in the acute correction group and from 10 to 43° (median, 21°) in the progressive correction treatment group. Residual postoperative deformity ranged from 1 to 8° (median, 3°) in the acute correction group and from 0 to 18° (median, 4°) in the progressive correction treatment group. Treatment duration ranged from 40 to 111 days (median, 64 days) in the acute correction group and from 40 to 87 days (median, 58 days) in the progressive correction group.

Discussion

Angular limb deformities often result from trauma to an active physis but may also result from metabolic disorders, osteomyelitis or septic physitis, or developmental disorders such as skeletal dysplasia (Carrig and others 1978, Marcellin-Little and others 1998). Deformities of the tibia, especially the distal portion of the tibia, are uncommon compared with distal antebrachial deformities (Ramadan and Vaughan 1979, Jevens and DeCamp 1993). Valgus deformities of the distal portion of the tibia have been sporadically reported in dogs and humans (Jeshrani and others 1980, Fox and Machon 1992, Jevens and DeCamp 1993, McCarthy 1998). The cause, pathogenesis and breed predisposition of this deformity is unknown but does not appear to result from trauma. The reported increased incidence in specific dog breeds (Shetland sheepdog, rottweilers) and the presence of bilaterally symmetrical deformities in several patients suggest that distal tibial valgus deformities are a developmental disease with a probable genetic influence. The overrepresentation of Shetland sheepdogs in this study coincides with previous reports of tibial valgus deformities in dogs (Vaughan 1987). In a previous report, a Shetland sheepdog, with bilateral valgus deformity of the distal portion of the tibia, distal fibular fractures and secondary distal fibular physeal closure were considered to be the causes of the deformity (Jevens and DeCamp 1993). Despite the small size of the fibula compared with the tibia, fibular anomalies have been shown to lead to tibia growth anomalies. Specifically, in humans, fibular hypoplasia, aplasia and congenital non-union have been reported to lead to tarsal valgus and varus angulations, tibial or fibular length deficits, abnormal tarsal development into a ball-and-socket joint, tarsal coalition, and foot and stance anomalies (Achterman and Kalamchi 1979, Wiltse 1972, Moon and others 1997, Frick and others 2001, Gonzalez-Herranz and others 2003). To assess whether fibular anomalies were present in the dogs in this study, we limited this study to patients without history of fracture or infection and we compared the relative size and position of the tibia and fibula in affected and non-affected limbs. The absence of fibular length deficit or proximal or distal fibular displacement suggests that a premature closure of the proximal or distal fibular physis is unlikely to be responsible for the development of the deformities described in this study. Therefore, it appears likely that the deformity results from an abnormal growth of the lateral aspect of the distal tibial physis and the absence of length deficits in affected tibiae suggests that this abnormal growth occurs relatively late in growth. This is in agreement with the fact that the dogs were also relatively old (median age at presentation, 9 months) compared with the dogs with antebrachial deformities treated within the same reference population (median age at presentation, 7 months, n=47, unpublished data, North Carolina State University, 1990 to 2005).

In the authors’ clinical experience, the most objective and useful presurgical measurements of tibial valgus are measurements of the orientation of the concave articular surface of the distal portion of the tibia and the measurements of the orientation of the pes made under sedation. By comparison, measuring valgus from the orientation of the pes on craniocaudal radiographs appears to underestimate the angulation because of the potential traction placed on the pes during radiography. Also, measuring valgus during weight bearing tends to overestimate the deformity because of the potential tarsal subluxation occurring during stance. It is also important to measure the length of both tibiae on mediolateral radiographs to rule out the presence of a length deficit.

The goal for the correction of angular limb deformities is to restore the mechanical axis and normal anatomic alignment of the limb. The association between limb malalignment and the subsequent development of osteoarthritis has been established and likely results from excessive forces placed on normal cartilage resulting from a shift in the mechanical axis of the joint (Johnson and Poole 1988, Kettelkamp and others 1988, Lovasz and others 1995, Marcellin-Little and others 1998). Experimental animal models creating or simulating a shift in the mechanical axis of the stifle, elbow and tarsus have confirmed the progressive development of osteoarthritis joints adjacent to the angled bone. In these models, the duration of mechanical derangement influenced the severity of cartilage damage and secondary osteoarthritis more than its magnitude (Ogata and others 1977, Johnson and Poole 1988, Lovasz and others 1995, Panula and others 1997). Prospective studies comparing long-term clinical results of different treatment modalities to conservative management are lacking. These studies would be made challenging by the high variability in size, weight and body fat content; deformity type; joint subluxation; and age of presentation of patients presented with limb deformities. In the absence of such specific knowledge, it seems logical to correct limb deformities primarily to improve limb function. The potential protection of joints is a secondary benefit of the correction of angular limb deformities.

The recommendation for corrective osteotomies for the patients included in this study was based on their functional impairment, the predisposition for the development of degenerative joint disease and the fact that the treatment of unifocal angular deformities often yields a good or excellent functional outcome (Johnson and Poole 1988, Lovasz and others 1995, Marcellin-Little and others 1998). The treatment of these deformities may be stabilised with plates, linear external fixation or circular external fixation (Fox and Machon 1992). HCEF is advantageous over the other fixation methods allowing limb lengthening and other postoperative adjustments. Although this was not assessed specifically in this study, differences in distraction fractionation between the patients treated in Parabiago and Raleigh did not appear to lead to differences in bone regenerate or patient morbidity.