Subluxation of the femoral head has an effect on magnitude and direction of the resultant femoral head force (Arnoczky and Torzilli, 1981). The purpose of this study is to relate age and hip laxity (HL) in puppies with the projected angle of inclination of the femoral neck (PAIFN).
During the period from 2002 to 2004, 102 Estrela Mountain Dogs were admitted to HV-UTAD, to evaluate hip dysplasia (HD). The population included 58 females and 44 males, younger than 1 year old, mean 6+/-2 months, and mean weight of 27+/-7 Kg. 23 of these dogs had a second evaluation after 1 year. All dogs were heavily sedated and the HL was evaluated with the Ortolani sign (OS) (Ginja and others, 2005). The symmetric axis method was used to measure the PAIFN in standard hip-extended ventrodorsal radiographic view (Rumph and Hathcock, 1990). We used the analysis of variance (ANOVA) to evaluate the influence of OS (101 positive and 103 negative hips) and age (50 dogs younger and 52 older than 6 months), in the measured PAIFN. The Bonferroni test was used to compare the PAIFN means. In the 23 dogs with two measurements linear regression analysis was used to evaluate the evolution of PAIFN in 23 normal and 23 dysplastic hips (positive OS or radiographic signs of HD). The mean PAIFN was significantly smaller in older (128.9°+/-3.9) than in younger (130.6°+/-3.8) dogs (p<0.01) and in dogs with positive (128.9°+/-3.9) versus negative (130.7°+/-3.8) OS (p<0.01). No interaction was observed between OS and age. The regression study indicated a non significant variation in PAIFN of normal hips (b=-0.07, p>0.4) and a statistically significant reduction in dysplastic hips (b=-0.28, p<0.001).
Hip subluxation alters the normal direction of compressive hip force and will lead the epiphyseal neck plate to a wedge-shaped growth. This pathologic growth pattern will tend to reorient the epiphysis and restore its perpendicularity to the new compressive force. The shift in direction of the femoral head force also supports alterations of periosteal growth, with preferential new bone deposition in lateral side of the femoral neck. All these early functional alterations result in the femoral neck realignment and PAIFN reduction.
References
1. Arnoczky, SP and Torzilli, PA. 1981. Am J Vet Res 42: 1581-5.
2. Ginja et al. 2005. 30th WSAVA Congress. Mexico. (In press)
3. Rumph, PA and Hathcock, JT. 1990. Vet Surg 19: 328-33.