Page 392 - Canine Lameness

364 20 Hip Region

(A) (C) (D)

(B)

Figure 20.11 Windup mechanism: juvenile animals with mild-moderate hip laxity (i.e. hip dysplasia)
may have (A) normal appearing standard radiographs with normal femoral head coverage (white arrow).
(B) PennHIP radiographs can be used to demonstrate pathologic laxity (white arrow). (C) This is explained
by the so-called “windup mechanism,” when standard leg extended ventrodorsal hip radiographs are
performed the joint capsule is twisted which forces the femoral head into the acetabulum artificially
improving the radiographic appearance of the hip joint. While (D) degenerative changes (black arrow) and
subluxation (white arrow) will become more obvious as the animal grows and the disease progresses, early
intervention (e.g. juvenile pubic symphysiodesis) may prevent such progression. As such, early diagnosis
(i.e. by 20 weeks of age) is crucial.

subjective evaluation of a ventrodorsal hip extended view for evaluation of hip conformation asso-
HIP REGION (all considered within normal limits), borderline, and mild, moderate, or severe (considered dys-
ciated with joint subluxation, joint congruity, and evidence of OA. A grade of excellent, good, fair
plastic) is assigned based on these secondary changes. Dogs must be 2 years of age or older to have
an official score given.
If subluxation is not obvious, PennHIP (Figure 20.12) may be performed as soon as 16 weeks of
age to more objectively evaluate laxity (Butler and Gambino 2017). PennHIP requires a certification
process, mandatory submission of the radiographs to a for-profit company (that will also evaluate
the radiographs and provide a report) and the use of a fulcrum device. The PennHIP report includes
a reported distraction index (DI) that is given relative to other dogs of the same breed. The report
also stratifies risk as low, mild, moderate, or high for developing OA based on the laxity of the indi-
vidual. The DI measures the maximal femoral head displacement from the center of the acetabu-
lum when a custom fulcrum device is used to place lateral stress to the proximal femur. It is
calculated by dividing the distance between the geometric center of the femoral head and geometric
center of the acetabulum by the radius of the femoral head. As the DI increases, so does the risk of
developing OA with age; dogs with a DI 0.3 are more at risk. PennHIP radiographs should always
be performed prior to testing for a positive Ortolani to avoid cavitation (Figure 20.12).
While the PennHIP distraction method appears to be highly predictive for the development of
OA, not all clinicians have access to the equipment and certified personnel required to perform this
series of radiographs. One method for performing distraction-stress radiography without the use of
a specific device was described by Flückiger et al. (1999). In dorsal recumbency femoral heads are
displaced manually in a craniodorsal direction and a subluxation index is calculated by comparing