There is a good summary here:
http://www.naiaonline.org/pdfs/longtermhealtheffectsofspayneuterindogs.pdf
Some other studies:
Here are some studies:
Endogenous Gonadal Hormone Exposure and Bone Sarcoma Risk1
• Dawn M. Cooley,
• Benjamin C. Beranek,
• Deborah L. Schlittler,
• Nita W. Glickman,
• Lawrence T. Glickman, and
• David J. Waters2
http://cebp.aacrjournals.org/content/11/11/1434.full
Early neutering may also increases risk of urethral sphincter incontinence in males (A. Aaron et al., Vet Rec. 139:542-6, 1996.)
Both neutered males and females have a 2-fold greater risk for developing bone tumors (osteosarcoma) compared to intact males and females (Ru et al., Vet J. 156:31-9, 1998.)
Some evidence suggests that there is a correlation between increased time for growth plate closure and incidence of HD in Labs (Todhunter et al. J. Am. Vet Assoc., 1997)
In addition, a study in 2004 in JAVMA (Spain et al. JAVMA 2004;224:380-387) showed that dogs spayed or neutered before 5 1/2 months had a significantly higher incidence of hip dysplasia than dogs spayed or neutered after 5 1/2 months of age.
There are a number of studies that suggest that those of us with canine athletes should be carefully considering our current recommendations to spay or neuter all dogs at 6 months of age or earlier. A study by Salmeri et al in 1991 (Salmeri et al JAVMA 1991;198:1193-1203) found that bitches spayed at 7 weeks were significantly taller than those spayed at 7 months, and that those spayed at at 7 months had significantly delayed closure of the growth plates than those not spayed (or presumably spayed after the growth plates had closed). The sex hormones close the growth plates, so the bones of dogs or bitches neutered or spayed before puberty continue to grow. This growth frequently results in a dog that does not have the same body proportions as he/she was genetically meant to. For example, if the femur is normal length at 8 months when a dog gets spayed or neutered, but the tibia, which normally stops growing at 12 to 14 months of age continues to grow, then an abnormal angle may develop at the stifle. In addition, with the extra growth, the lower leg below the stifle becomes heavier (because it is longer), causing increased stresses on the cranial cruciate ligament. This is confirmed by a recent study showing that spayed and neutered dogs have a higher incidence of CCL rupture (Slauterbeck JR, Pankratz K, Xu KT, Bozeman SC, Hardy DM. Canine ovariohysterectomy and orchiectomy increases the prevalence of ACL injury. Clin Orthop Relat Res. 2004 Dec;(429):301-5).
