First steps in the development of an ovine proximal phalanx fracture and bone defect model: a study of animal welfare and bone healing

Abstract

There is a need for large-animal fracture models focusing on small tubular bones, as existing models typically involve major weight-bearing bones and often rely on restrictive suspension systems that raise significant animal welfare concerns. This study presents a novel in vivo sheep model targeting the proximal phalanx, designed to enable natural movement and social housing whilst supporting the investigation of fracture fixation and bone healing. Eleven skeletally mature Texel sheep were included; four underwent bilateral ostectomies with 3 mm or 6 mm defects, and seven received an osteotomy. A hoof block was used to offload the affected digit, enabling the sheep to move freely and to be housed in pairs. Bone healing was assessed using standardised radiographic scoring based on images obtained intraoperatively, at week 1 and 2, and hereafter every second week. Hard callus thickness was assessed at the end of the study period by a computed tomography-based method. Animal welfare was monitored through repeated clinical evaluations, two pain scoring systems (a validated sheep facial pain expression scale and a novel composite behavioural pain score), and biomarkers of inflammation, including serum amyloid A and haptoglobin. The ostectomy group showed frequent implant failure and limited healing, particularly in the 6 mm ostectomies, whilst the osteotomy group demonstrated relatively better stability and more consistent healing. Pain scores peaked shortly after surgery and again following withdrawal of analgesia but remained generally low. This study presents a novel, welfare-consciousness bilateral ovine proximal phalanx fracture model that challenges osteosynthesis stability. The integrated multimodal welfare assessment highlights the importance of objective pain and stress monitoring and advocates for the routine inclusion of quantitative welfare parameters alongside bone healing outcomes in translational orthopedic research.

Keywords: biomarkers; bone healing; large animal model; pain scores; sheep; small tubular bones.

Figure 1

Overview of the ovine thoracic limb bilateral single toe (OBST) fracture model. (A) The ostectomy group showing (I) a schematic drawing of the bilateral intervention, (II) the application of the cutting guide, (III) the ostectomized bone segment of 3 mm and 6 mm and (IV) the ostectomized proximal phalanx osteosynthesized with 6-hole locking plate. (B) The osteotomy group showing (I) a schematic drawing of the bilateral intervention, (II) the osteotomized proximal phalanx osteosynthesized with 5-hole locking plate, the 4.5 mm drill-hole evident (blue arrow), and (III) radiograph depicturing the osteotomy line, drill-hole (blue arrow), metal-plate and bicortical screws. (C) Coaptation modalities of a left thoracic limb; (I) hoof block and elastic bandage, and (II) a half limb walking cast with no hoof block.

Figure 2

Pain score results from the sheep with ostectomies (n = 4). The results are presented as cumulative scores, with the y-axis representing the highest attainable score (4 x the maximum score), insert graphs visualising results in detail. Note that these graphs represent only 4 animals (important for comparison with cumulative data from 7 sheep in Figure 3). (A) Sheep pain facial expression scale [SPFES; (38)] and (B) clinical severity score [CCS; (21)]. 0: All sheep scored 0.

Figure 3

Pain score results from the sheep with osteotomies (n = 7). The results are presented as cumulative scores, with the y-axis representing the highest attainable score (7 x the maximum score), insert graphs visualising results in detail. Note that this graph represent data from 7 sheep (in comparison to the 4 sheep in Figure 2) (A) sheep pain facial expression scale [SPFES; (38)] and (B) the ovine orthopaedic pain behaviour score (OOPBS).

Figure 4

Lameness scores extracted from the clinical severity score from the sheep with ostectomies (n = 4) and the ovine orthopaedic pain behaviour score from the sheep with osteotomies (n = 7). The results are presented as cumulative scores, with the y-axis representing the highest attainable score, insert graphs visualising the results in detail.

Figure 5

Biomarkers measured from the sheep (n = 4) included in the ostectomy group (A–C) and from the sheep (n = 7) included in the osteotomy group (D–F). Saliva cortisol (A,D) and plasma SSA (B,E) and haptoglobin (C,F). Solid line: median. SAA, Serum amyloid A.