Page 1067 - Adams and Stashak's Lameness in Horses, 7th Edition

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CHAPTER

Lameness in the Young Horse

THE PHYSIS/PHYSEAL FRACTURES/PHYSITIS

Dane M. TaTarniuk, Troy n. TruMble, anD Gary M. baxTer

THE PHYSIS contours of the bone of an adult as bone development
reaches the margins of the cartilage model (Figure 10.3).
Growth and Development of the Musculoskeletal The epiphysis forms early in life at one or both ends
System of long bones between the growth plate and the articular
During fetal development, equine long bones have cartilage. These articular–epiphyseal complexes form in
two main centers of ossification. The primary ossifi­ a manner similar to ossification of the cartilage models
cation center develops first and forms the diaphysis and of the small cuboidal bones. This ossification mostly
metaphysis of the bone, while the secondary ossifica­ increases the size of the epiphyses but contributes very
tion center will form the epiphysis. In between these little to the length of the bone (Figure 10.2). The cellular
two centers of ossification is a specialized, highly cellu­ events consist of vesiculation and chondrocyte death
lar region referred to as the physis or growth plate. with calcification of matrix, invasion of vessels, and par­
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Most long bones have a physis at each end of the bone, tial resorption and ossification. Eventually, a subchon­
whereas others have only one, such as the third dral bone plate forms protecting the articular cartilage
metacarpal/metatarsal bone (distally) and the proximal portion that persists throughout life.
and middle phalanges (proximally). The function of the
growth plate is to increase longitudinal bone growth
via endochondral ossification, a process where rapidly Endochondral Ossification of the Physis (Growth Plate)
differentiating and maturing cartilage cells get replaced The metaphyseal growth plate or physis has a
by bone. 49 characteristic cellular architecture from birth to maturity
There are two types of growth plates: discoid and that allows for simultaneous longitudinal growth and
spherical. Discoid growth plates are located in long ossification of bone. The cartilage cells of the growth
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bones between the metaphysis and the epiphysis plate can be divided into a number of zones, which vary
(Figure 10.1). In addition, a type of discoid physis is pre­ in height and cell number, histological appearance, and
sent distal to an apophysis (specific type of epiphysis cellular function. They include the resting/reserve,
that is subject to tensile rather than compressive forces) proliferative, hypertrophy, provisional calcification, and
on the olecranon process (Figure 10.1), calcaneal tuber, ossification zones. 49,50 On the outer periphery of the
and tibial tuberosity. These physes contain greater growth plate is a perichondrial ring of collagen fibers
amounts of fibrocartilage to withstand tensile forces. oriented vertically, obliquely, and circumferentially often
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Spherical physes are located in the small cuboidal bones referred to as the ring of La Croix. It encircles the
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of the carpus and tarsus (Figure 10.2). These growth growth plate and extends from the metaphyseal perios­
plates develop into bones by centrifugal expansion teum to epiphysis (Figure 10.4). Its main function is to
around a central cartilage core. The cartilage core provide mechanical support for the growth plate while
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begins to ossify in the center and gradually assumes the also acting to constraint the circumferential growth of

Adams and Stashak’s Lameness in Horses, Seventh Edition. Edited by Gary M. Baxter.
© 2020 John Wiley & Sons, Inc. Published 2020 by John Wiley & Sons, Inc.
Companion website: www.wiley.com/go/baxter/lameness
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