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Originally Posted by Maximo  Once again, what evidence is there that points to causation vs correlation? I think there are environmental/human factors that would be very difficult to measure, especially for small breeds. |
The article addresses these issues in the conclusions section:
One potential source of bias in this study is that only dogs whose owners sought care for lameness were included. This potential source of bias is inherent in all retrospective studies of clinic records. Of particular concern is the possibility that some dogs in our region (e.g., strays or pets of negligent owners) never receive veterinary care of any kind, including for spaying, neutering, or treatment for lameness. If such animals were a large segment of the dog population in our region, it would tend to exaggerate the apparent prevalence of ACL injury in spayed and neutered animals in our study because a population of nonspayed, nonneutered, lame dogs was not represented. However, one also must consider that our region includes a substantial population of highly prized, nonspayed, nonneutered dogs specifically bred for show and for working, especially hunting. Owners of such animals typically seek veterinary care routinely, and especially for conditions that are highly symptomatic, such as ACL injury. Consequently, this population might be over- represented in our study, and thereby would tend to exaggerate the apparent prevalence of ACL injury in nonspayed, nonneutered animals, in opposition to the effects of the stray and neglected population. Regardless, both of these populations are likely to be small in comparison to the overall dog population in our region, and therefore unlikely to affect significantly the conclusions of this study.
Comparisons of mechanism of injury between the dog and human are difficult. The anatomy of the dog knee (stifle joint) is different from that of the human knee. The active range of motion of the dog stifle is 46°–142°,14 whereas that of the human knee is 0°–160°. Dogs have a four-legged gait, so the dynamics of running and cutting are different from humans. These differences suggest the mechanism of injury in dogs is not the same as in humans. Nevertheless, the mechanism of injury is likely to be similar, and most, or all, of the external biases affecting ACL injury introduced by human behavior are minimized in the dog.
The effect of ovariohysterectomy and orchiectomy on the dog is complex and results in various body changes in the animal far greater than an effect seen solely from the absence of one sex hormone. For example, bone density decreases secondary to the reduction of the influences of sex hormones on calcium deposition into the skeleton. Despite the effect on bone, none of the injured animals had a ligament-bone avulsion at the time of surgery. Whether ovariohysterectomy or orchiectomy affect weight gain is controversial, but one study showed no significant increase.21,22 Additionally, ovariohysterectomy or orchiectomy could affect the behavior of the dog.21,22 However, an ovariohysterectomized or orchiectomized dog most likely would become less aggressive, less active, or both, probably decreasing the risk for ACL injury.21,22 Although we cannot predict all of the effects that could result from canine ovariohysterectomies or orchiectomies, many of the effects are likely to decrease injury risk.
The effect of decreased sex hormones on the ACL of the dog is not known. However, alterations in sex hor- mones may affect the size, shape, or material properties of the ACL. Anterior cruciate ligament remodeling is affected by loads placed on the ligament (Wolff’s Law). The remodeling of many soft tissues, including the ACL,1,5,7,13,15,16 also is influenced by sex hormones. In humans, elevated levels of estrogen decrease collagen pro- duction of the ACL in tissue culture. Additionally, expres- sion of an ACL remodeling gene is increased, on average, in human females. Gender differences in ligament remod- eling presumably would affect the size, shape, or internal structure of the ACL, thereby affecting its ability to sustain loads without rupturing. Therefore, it is possible that by altering ligament remodeling, a decrease in sex hormones would affect the predisposition of the ACL to injury.
Ovariohysterectomy and orchiectomy increased the prevalence of ACL injuries across sizes and breeds of dogs. These findings have established an alternative model by which the effect of gonadal gender on ACL injury can be studied, and are consistent with the view that ACL injury in humans is, in part, related to gender differences in hormone production and presentation.