OBJECTIVES

The fragmented medial coronoid process (FCP) of the ulna and especially a small fragment of it interposed between radius and ulna is the most important manifestation of elbow dysplasia in retrievers in The Netherlands. The cause of the disease is still unknown but one of the factors is primary or secondary mechanical overloading. The medial coronoid process (MCP) is due to its anatomical position subjected to various forces: by the medial condyle of the humerus during stance and walking, by the radius during rotation in the radio-ulnar joint, and by tension of the annular ligament. The trabecular orientation and density will give an indication of direction of the forces the bony part is subjected to, since forces directly affect a bone's cancellous structure (Wolff's law). These parameters can be calculated from a three-dimensional data set, obtained by microcomputed tomography (microCT). The aim of this study is to describe the trabecular bone structure of the MCP and its development at different ages in order to provide data about the normal loading pattern of the MCP and the possibility of (regional) over-loading during the growth of the dog.

MATERIALS

The MCP's of ten Golden retrievers aged 4, 6, 8, 10, 13, 16, 18, 20, 22 and 24 weeks, that were not lame and had clinically healthy elbow joints, were dissected after euthanasia. Cylindrical cores were cut using a hollow coring bit of 17 mm, including the whole MCP, its articular surface and the attachment of the annular ligament. MicroCT scanning was performed with a 34-micron voxel size. With the obtained data 3D- reconstructions were calculated. The following parameters were evaluated: bone volume (BV) fraction (BV/TV, TV = total volume), trabecular thickness (Tb.Th; mm), trabecular separation (Tb.Sp; mm), mean trabecular number (Tb.N; No.mm-1), and the degree of anisotropy DA13, DA12, DA23 as a measure for the trabecular direction. Differences between ages were analyzed with the univariate analysis (General Linear Model); post hoc Bonferroni tests were used to identify age differences in case of a significant result. Differences between regions were tested using the Independent-Samples T Test. All tests were conducted with SPSS 10.0 software (SPSS Inc.); at p<0.05 differences were considered to be statistically significant.

RESULTS

At dissection all ten elbows appeared normal with no joint extension or visible damage to the articular cartilage or bone. However at the reconstructions of the whole MCP it showed that the dog of 20 weeks of age had a fracture in the subchondral bone at the position where an interposed fragment would be expected. Regarding the remaining ages: bone volume fraction and trabecular thickness were significantly smaller at the ages up until 13 weeks, the number of trabeculae was significantly higher, while the intertrabecular distance remained constant at all the ages studied. The main alignment of the trabeculae (degree of anisotropy, DA13) was orientated at an angle of approximately 23 degrees with the proximo-distal axis of the ulna, (perpendicular to the articular surface of the MCP) and was present from the age of eight weeks on. The second alignment of the trabeculae (DA12) was orientated at an angle of approximately 30 degrees with the cranio-caudal axis of the MCP, in the direction of the tensile forces evoked by the attachment of the annular ligament. No distinct alignment was found in the medio-lateral direction. Furthermore no differences in trabecular structure between the inner and outer half of the MCP were found.

In the 20 weeks old dog the bone volume fraction was significantly higher, the trabeculae were significantly thicker and the intertrabecular separation was smaller. The bone had a more cortical appearance.

CONCLUSION

The growth of the MCP brings about the same changes in (histo) morphometric bone parameters as seen in the growth of long bones, although the orientation of the trabeculae seems to develop earlier. The main forces on the MCP, concluded from its trabecular pattern are transmitted by the medial condyle and by the annular ligament. There is no distinct alignment along the medio-lateral axis, the direction in which the radio-ulnar joint provokes stresses on the ulna. Therefore it is not likely that, if the radius and ulna develop in a normal way, the forces at the radio-ulnar joint can cause a FCP.

The denser structure of the FCP can primary be responsible for the fracture or can secondary develop because of an underlying disease. The cause of the condensation cannot be cleared with microCT, so further histological analysis has to be undertaken. It is however very well possible that the denser structure at the base of the MCP at radiographs is caused by a denser internal structure in case of a FCP.