Suspensory ligament or interosseous muscle? Histology of the interosseous muscle of South American Camelids

The interosseous muscles (IOM) have evolved to largely collagenized structures in many large grazers, including South American Camelids (SACs). SACs include lama (Lama glama), alpaca (Vicugna pacos), guanaco (Lama guanicoe) and vicuna (Vicugna vicugna). This study reports on a comparative histomorphological evaluation of the IOM in these four species. First, a descriptive qualitative analysis of the tissue composition within the physiological IOM clearly demonstrates that the IOM of all SAC species is predominantly tendinous. However, we can always find a small amount of striated muscle fibres supporting the claim that the term "interosseous muscle" should be preferred over the denomination "suspensory ligament". Second, we quantify the ratio of muscle tissue (MTr) occupying the cross-sectional area (CSA) of 80 limbs at three levels and examine it in relation to limb, sample localization and species. Values for MTr are highest at the middle level, ranging from 0.4% to 15.2% with a mean of 3.4% ± 3.2%. We also detect notable amounts of muscle tissue at the distal sample level in the area of the branches of the IOM. Of all SACs, alpacas present the highest MTr. Lamas have a significantly larger MTr than guanacos. These results suggest the influence of domestication and/or physical activity on the muscle tissue content of the IOM of SACs. Amounts and distribution of muscle, adipose and lose connective tissue vary greatly for each sample localization and within individuals of one species. This may complicate the interpretation of MRI images of SACs' IOM for diagnostic purposes.

Medical management of an osseous sequestrum in an alpaca cria

CASE DESCRIPTION

A 2-month-old 12.0-kg (26.4-lb) sexually intact male alpaca was evaluated for a 1-week history of progressive forelimb lameness with no known history of trauma.

CLINICAL FINDINGS

The cria had toe-touching lameness in the right forelimb with a firm swelling at the distal dorsomedial metacarpal region. Signs of pain were elicited on palpation of the swollen region. There was no associated wound or draining tract. Radiographic examination revealed soft tissue swelling and osteomyelitis of the distal portion of the third metacarpal bone with a possible medial cortical sequestrum.

TREATMENT AND OUTCOME

The cria was hospitalized and treated with meloxicam (1 mg/kg [0.45 mg/lb], PO, q 72 h) and ceftiofur sodium administered SC (2.2 mg/kg [1 mg/lb], q 12 h for 8 days) and by means of regional limb perfusion (1.25 mg/kg [0.57 mg/lb], IV, q 48 h for 8 days). Lameness and swelling improved, and the cria was discharged from the hospital with meloxicam (1 mg/kg, PO, q 72 h for 2 weeks) and ceftiofur crystalline free acid (1.5 mg/kg [0.68 mg/kg], SC, q 5 d for 2 weeks). At a recheck examination 17 days later, there was radiographic evidence of a well-defined 3.4 × 0.3-cm osseous sequestrum in the distal aspect of the affected third metacarpal bone. The owner declined further treatment and elected to monitor the cria at home. One year later, radiographic examination revealed nearly complete resolution of the sequestrum.

CLINICAL RELEVANCE

Results for this patient suggested that osseous sequestra in some camelids may resolve following medical treatment without surgical intervention.

Comparative anatomic and morphometric examination of the interosseous muscle, sesamoid ligaments and flexor tendons of the fetlock in South American camelids

This study presents a comparative morphological and morphometrical examination of the interosseous muscle (IOM), digital flexor tendons and associated structures of all four species of South American Camelids (SACs): Lama (Lama glama), alpaca (Vicugna pacos), guanaco (Lama guanicoe) and vicuna (Vicugna vicugna). Our findings show that the lamoid interosseous muscle and digital flexor tendons are very similar between species and differences, which have been reported previously, are possibly due to diverging examiner interpretation. Lumbricalis muscles were mostly found as single muscles in this study. The definition of the terminal and dorsal branches of the interosseous muscle is critically revised. The analysis of morphometric data indicates that the size of the individual has a substantial influence on the cross-sectional area (CSA) of the examined structures. Metacarpal/metatarsal bone width is a main predictor of CSA, while the factor species only shows significance in specific sample points. For lama and alpaca, no differences in CSA in relation to species as factor were found. The IOM shows the largest CSAs of the three examined structures in all species, which implies an important load-bearing function. Considerable individual variation remains unexplained by the mentioned factors. This lowers the prospect of CSA reaching diagnostic importance in SAC fetlock pathology.