Anatomy

Developmental orthopedic diseases and osteoarthritis are common diseases seen in canine practice and account for 25% of patient visits to veterinarians. Of those visits for osteoarticular disorders, 70% are due to diseases of the appendicular skeleton, 20% are probably due to dietary origin and more than 22% occur in dogs under one year of age (Richardson et al., 1997; Johnson et al., 1994). Large breed dogs account for the majority of cases however any size dog can be affected.

The development of these skeletal diseases may be due in part to dietary management. Nutritional excesses and deficiencies can contribute to canine osteoarticular disorders. In many cases, optimal intake of a balanced diet can prevent or at least diminish the severity of the diseases. For a few diseases, dietary correction alone is sufficient to restore skeletal integrity.

Herman HAZEWINKEL
DVM, PhD, Dipl ECVS, Dipl ECVCN

Dr. Hazewinkel works in the orthopedic group of the Department of Clinical Sciences of Companion Animals at Utrecht University in the Netherlands. He did his PhD studies on the influence of calcium intake on the skeletal development in young dogs, becoming a specialist in companion animal surgery (RNVA), Diplomate of the European College of Veterinary Surgeons, and Diplomate of the European College of Veterinary and Comparative Nutrition. Since 1998 he has occupied the chair dedicated to diseases of skeletal development in companion animals and gives guidance to research studying the influence of different aspects of nutrition on skeletal development, osteoarthrosis, and research of the hereditary and traumatic skeletal diseases.

He is president of the International Elbow Working Group, past-president of the European Society for Veterinary Orthopedics and Traumatology, member of the Hereditary Committee of the WSAVA and other specialty groups examining companion animal orthopedics and nutrition.

Jocelyn MOTT
DVM, Dipl ACVIM

Jocelyn Mott received her doctor of veterinary medicine from the Western College of Veterinary Medicine, Saskatoon, Saskatchewan, Canada. She then completed a small animal internship at Oklahoma State University followed by a small animal internal medicine residency at the University of Wisconsin - Madison. Dr. Mott is a Diplomate of the American College of Veterinary Internal Medicine. She has spent several years in private specialty practices across the USA (Massachusetts, Texas, Florida and California).

1. Anatomy

Bone Composition

Bone is a specialized form of connective tissue with a complex chemical and physical composition (Table 1). Apart from its cellular fraction (10%) and the water phase (25%), it is composed of an organic matrix and a mineral phase. The cellular fraction includes osteoblasts (organic matrix-forming cells) and osteoclasts (calcified matrix-resorbing cells). The organic matrix is composed of 90% collagen fibers with a high content of hydroxyproline and 10% aminopolysaccharides, non-collagen proteins and a small quantity of lipid. The mineral phase encompasses about 65% of the bone volume, mainly in the form of hydroxyappatite crystals and amorphous calcium phosphate, as well as small quantities of other elements. Of the total body calcium and phosphorus, 99% and 80%, respectively, are present in the skeleton.

Calcium Cartilage Composition

There are striking differences between the organic and inorganic composition of the main constituents of the skeleton, i.e., bone and cartilage (Table 1). The difference between bone and cartilage is mainly the flexibility (and therefore the water content), the lack of mineral deposition, and the difference in collagen elements. Cartilage contains chondroblasts, proteoglycans, and collagen. The latter are anchored in the subchondral bone in the tide-mark.

Proteoglycans are composed of glycosaminoglycans (GAGs) and a core protein called aggrecan. Aggrecan is an important proteoglycan in cartilage with keratin sulphate and chondroitin sulphate being the most important GAGs. About 200 aggrecan molecules are bound via a glycoprotein to a hyaluran molecule which binds a large quantity of extracellular water (Figure 1).

Figure 1. Schematic representation of aggrecan within articular cartilage. Large proteoglycan molecules are linked to long chains of hyaluronic acid and stabilized by two or more link proteins to form the large proteoglycan aggregates found in cartilage.

Collagen molecules in cartilage contain large amounts of hydroxyproline and hydroxylysine. The molecules form a triple helix structure, bound to fibrils and these form fibers. Cartilage does not contain blood or lymphatic vessels. With loading, extracellular water is pressed out of the cartilage until the decreased diameter of the pores and the increased negative charge prevents further escape of water, whereas at unloading fresh water and nutrients enter the cartilage.