Lengths, internal and external diameters, cortical thicknesses, distal proportions of cancellous bone and other useful features were all documented in a group of skeletally mature koalas. Although CT scans were previously employed for morphometric analysis of the koala nasal cavity,(10) detailed appendicular skeletal morphometry has remained undocumented. Using micro-CT in this study provided much smaller slice capabilities for reconstructed images, allowing for more precise descriptive analysis aimed to benefit future clinical applications.
The medullary cavity of the diaphysis tapers distally, and throughout the entire bone the dimensions of the cavity are greater in the mediolateral plane. This is important for orthopaedic research as failure rates of implants such as the interlocking nail are dependent on their design and size.(19) In mature koalas, should implant stabilisation be implicated in fracture repair, the size and shape of the implant should cater for the tapering internal diameter to optimise stability and promote healing. The humeral medullary cavity also allows implants to have a marginally greater mediolateral than craniocaudal diameter. Intramedullary implants such as the interlocking nail are typically directed normograde through the medullary cavity, embedding into the distal cancellous bone for stability.(20) This study showed the distal cancellous bone fills the cavity approximately 79% down the length of bone. This will guide surgeons when developing surgical implant length recommendations for future fracture repairs. Other important details regarding the shape of the koala humerus that must be considered when developing orthopaedic techniques is the craniocaudal flattening of the epiphysis and the caudal curvature of the distal humerus. These factors may complicate how conventional implants embed into the distal cancellous bone. The flattening suggests the further distal the implant, the less craniocaudal space there will be to accommodate the implant, and the curvature of the bone suggests that anatomically pre-contoured implants might be required.
Diaphyseal cortical proportions were relatively homogenous with a very marginal increase in the thickness of more distal bone. From an orthopaedic perspective, this result suggests the diaphyseal cortex should have minimal variation in strength throughout its length.(11) Although slight, the increase in distal diaphyseal thickness may suggest the distal diaphysis can withstand slightly higher forces than proximal bone. However, it is important to note that bone strength does not only depend on cortical thickness, but also other structural factors (bone diameter and shape) and material properties of the bone (density and porosity).(21) Cortical thickness and bone shape likely reflect the origin and insertion of surrounding soft tissue attachments. Regions of cortical and cancellous bone have been shown to remodel following dynamic physiological stress and external forces acting on the bone.(22–24) No distinct osseous features course medially as the deltoid and supinator ridges do laterally. The increased surface area provided by the deltoid ridge means that dynamic tensile forces associated with the pectoralis major and deltoid insertion sites(8) can be distributed over a greater area requiring less focussed cortical strength. Similarly, at the distal diaphysis the supinator ridge marks the origins of the extensor carpi radialis longus and supinator longus muscles.(8) The medial surface has no bone protuberances and a comparatively thicker cortex. The thicker medial cortex could be useful for screw placement, as thicker cortices have greater screw loading strength.(20, 25) The relatively uniform surface may also be ideal for minimally invasive osteosynthesis, should plate fixation be indicated.(20) Knowledge of how surrounding soft tissue structures relate to cortical thickness can help develop optimal surgical approaches to the koala humerus based on muscular attachment sites and bone strength.
This descriptive study on koala humeral morphometry has several limitations that need to be considered. Measurement parameters calculated in this study were not paired with koala sex, weight, or age. Analysis between cohorts defined by these variables may provide useful points of comparison when developing surgical recommendations. For example, male koala body size and weight is typically greater than females.(26, 27) It is therefore possible that male humeral parameters could be larger than female measurements. Additionally, the dataset analysed was limited to a cohort of six mature, cadaveric koalas and may not be representative of cohorts that include geriatric or juvenile individuals.
Radiographic inspection of each humerus prior to inclusion in the study meant any immature or skeletally diseased animals were excluded. Therefore, this study provides useful anatomical details for orthopaedic researchers investigating treatment of humeral fractures in mature koalas. Despite this, individual koala reproductive and nutritional status remained unknown. These factors are known to influence bone development, quality and cancellous bone volume.(11, 26) Being from an unmonitored wild population establishing these details can be inconsistent and is a limitation of this study.
Only right humeri were used in this study, so potential variation between left and right sides should be considered. Preferential limb use in koalas is unknown but has been demonstrated in other marsupial species including the brushtail possum,(28) and red-necked wallaby.(29, 30) If there is continual preference for a particular limb, bone size and quality may change in response to additional dynamic force remodelling.(23, 24) Studies have investigated koala muscle mass proportions between hind and forelimbs but not between individual limbs.(5) If preferential limb treatment is present in koalas it is unknown whether the effect would be substantial enough to make a difference to healthy koala bone morphometry.
Fracture location has been shown to be prognostic in influencing whether koalas can be successfully rehabilitated and released.(2) However, no previous studies have described the frequency or distribution of fracture types in specific koala bones. This may be the subject of future research as different repair techniques may be indicated based on the nature, location and characteristic of the presenting fracture.(31) Different fracture locations along the humerus provide unique biomechanical considerations that may influence the treatment type.