Osteoporosis: Epidemiology, Histology, Bone Remodeling, and Classification

Peter A. Huijbregts, DPT, OCS, FAAOMPT

Abstract: Osteoporosis and decreased bone mineral density affect a large proportion of Americans of both sexes. Physical therapists play a role in screening for undiagnosed osteoporosis, and in prevention and treatment of osteoporosis. Bone mineral density is determined by the activity of the cells contained in bone. These cells react to demands on calcium and phosphate metabolism, hormonal influences, and mechanical loading. Osteoporosis can be classified as primary or secondary.

Key Words: Osteoporosis, Epidemiology, Histology, Remodeling, Classification

Decreased bone density or osteopenia is pathogno monic for osteoporosis1,2. Despite the decreased density, the osteopenic bone in osteoporosis is normally mineralized1-3. This distinguishes osteoporosis from osteomalacia, a condition with which it is commonly confused; in osteomalacia, the bone matrix is insufficiently mineralized1,4. One way to assess bone mineral density (BMD) is by dual-energy X-ray absorptiometry (DEXA). According to Glase and Kaplan1, the World Health Organization (WHO) has used this diagnostic test to establish criteria for the classification of osteoporosis (Table 1). The physical therapist has three distinct roles with regards to patients at risk for or diagnosed with osteoporosis or osteopenia.

Our first role pertains to screening patients for osteoporosis and osteopenia. The fracture threshold is a hypothetical concept representing a BMD below which fracture as a result of unspecified external forces becomes more likely. Some physical therapy interventions may generate mechanical forces that exceed the fracture threshold in a patient with decreased BMD. In this era of direct access to physical therapy services and limited patient contact time with the primary-care physician due to managed care constraints, the physical therapist, therefore, needs to be able to screen patients for a low BMD. Being able to identify patients at risk for low BMD will not only allow the therapist to appropriately choose intervention parameters but also enable a timely referral to medical colleagues.

Our other two roles deal with prevention and treatment. The Guide for Physical Therapist Practice5 contains a preferred practice pattern for primary prevention and risk-factor reduction for skeletal demineralization. It describes two treatment goals. One is having the patient who is at risk for low BMD maintain a density above the fracture threshold. The other goal is having the patient with identified low BMD reverse the demineralization process and achieve BMD above the fracture threshold.

This article is Part I of a two-part series. The goal of this part is to increase the physical therapist’s knowledge of the epidemiology of osteoporosis, the histology of bone, the influences on bone remodeling, and the classification of osteoporosis. This knowledge will serve as a basis for the follow-up article, which deals with the more practical aspects of diagnosis and treatment. Together these two articles will enable the physical therapist to effectively screen patients for undiagnosed osteoporosis and to develop appropriate treatment programs for those patients at risk for or diagnosed with osteoporosis.

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