SI luxation landmarks and fixation reference

The original sacroiliac (SI) note combines iliac landmarks, screw sizes and fixation outcomes. The iliac glide hole and the sacral drill hole use different coordinates.

In dogs, the iliac landmark is midway through the caudal half of the tuber sacrale: about 75% from its cranial end and one-third of local iliac height down from dorsal. The length measurement runs parallel to the dorsal plane. In cats, the reported iliac coordinates are 69% of tuber-sacrale length from cranial and 52% of iliac height from dorsal. The feline length measurement follows the iliac spine. The recall axes are not drawn to scale.

The book lists 2.0 or 2.7 mm screws for cats and small dogs, 3.5 or 4.0 mm for medium dogs, and 4.5 or 6.5 mm for large dogs. These are size examples, not a continuous range or a rule for every patient. Plan diameter and trajectory for the available bone. Sacral purchase is measured within the sacrum; it excludes the iliac thickness and screw head. The historical guidance seeks at least 60% of sacral width. Total screw length also includes the iliac passage.

One statistic needs correction. The note and book give 38% loosening for purchase below 60%. The cited 1985 abstract reports 48% (10/21) below 60%, versus 7% (1/14) at 60% or more. Its 38% is 11/29 re-radiographed dogs overall. These reported subgroup counts are not pooled with that overall dog count. In a separate 1999 series of 13 dogs, mean sacral purchase was about 79%, with no loosening observed. That does not establish a 79% threshold or zero risk. Loosening does not automatically mean poor function.

References

Chris Wood’s notes; Johnston & Tobias, Veterinary Surgery: Small Animal, 2nd ed., ch. 56, PDF pp. 2259–2263, Fig. 56.25; Burger et al. 2004, doi:10.1055/s-0038-1632803; DeCamp & Braden 1985, doi:10.1111/j.1532-950X.1985.tb00841.x; Tomlinson et al. 1999, PMID10338164. Primary abstracts read; full papers not obtained.