Bone circulatory disturbances in the development of spontaneous bacterial chondronecrosis with osteomyelitis: a translational model for the pathogenesis of femoral head necrosis

Abstract

This review provides a comprehensive overview of the vascularization of the avian growth plate and its subsequent role in the pathogenesis of bacterial chondronecrosis with osteomyelitis (BCO, femoral head necrosis). BCO sporadically causes high incidences of lameness in rapidly growing broiler (meat-type) chickens. BCO is believed to be initiated by micro-trauma to poorly mineralized columns of cartilage cells in the proximal growth plates of the leg bones, followed by colonization by hematogenously distributed opportunistic bacteria. Inadequate blood flow to the growth plate, vascular occlusion, and structural limitations of the microvasculature all have been implicated in the pathogenesis of BCO. Treatment strategies have been difficult to investigate because under normal conditions the incidence of BCO typically is low and sporadic. Rearing broilers on wire flooring triggers the spontaneous development of high incidences of lameness attributable to pathognomonic BCO lesions. Wire flooring imposes persistent footing instability and is thought to accelerate the development of BCO by amplifying the torque and shear stress imposed on susceptible leg joints. Wire flooring per se also constitutes a significant chronic stressor that promotes bacterial proliferation attributed to stress-mediated immunosuppression. Indeed, dexamethasone-mediated immunosuppression causes broilers to develop lameness primarily associated with avascular necrosis and BCO. Prophylactic probiotic administration consistently reduces the incidence of lameness in broilers reared on wire flooring, presumably by reducing bacterial translocation from the gastrointestinal tract that likely contributes to hematogenous infection of the leg bones. The pathogenesis of BCO in broilers is directly relevant to osteomyelitis in growing children, as well as to avascular femoral head necrosis in adults. Our new model for reliably triggering spontaneous osteomyelitis in large numbers of animals represents an important opportunity to conduct translational research focused on developing effective prophylactic and therapeutic treatments.

Keywords: femoral head necrosis; osteomyelitis.

Figure 1

Diagram depicting the arterial supply to a growing leg bone. The epiphyseal vascular supply (ev) courses through epiphyseal vascular canals (ec) within the hyaline zone (hy) of the epiphysis (e). Terminal branches of some ev form penetrating epiphyseal vessels (pev) that supply the epiphyseal side of the growth plate (gp) or physis. The nutrient artery (na) penetrates the diaphysis (d) at the nutrient foramen (nf) and divides into ascending and descending branches (ana, dna) that subdivide repeatedly to form metaphyseal vessels (mv) within the metaphysis (m). The mv supply the metaphyseal vascular plexus (mvp) on the metaphyseal side of the gp. Additional features include: a, articular zone of epiphyseal cartilage; eb, endosteal bone; eo, endosteal arterioles; mc, medullary cavity; pb, periosteal bone; po, periosteal arterioles. Micro-anatomical terminology is consistent with the established anatomical nomenclature for avian bones (Beaumont, ; Lutfi, ,; Wise and Jennings, ; Howlett, , ; Hunt et al., ; Duff, ; Howlett et al., ; Thorp, , ; Ali, ; Farquharson and Jefferies, 2000).

Figure 2

Diagram depicting the proximal head of a growing leg bone. The epiphyseal vascular supply (ev) courses through epiphyseal vascular canals (ec) within the hyaline zone (hy) of the epiphysis (e), or through junctional canals (jc) directed toward the growth plate (gp) or physis. Branches of the ev terminate as epiphyseal vascular capillary complexes (evc) within the hyaline zone, or become penetrating epiphyseal vessels (pev) that terminate as a penetrating vascular capillary plexus (pvp) supplying the resting zone (rz), proliferating zone (pz), and prehypertrophic zone (phz, also known as the maturing zone) of the gp. The ascending branch of the nutrient artery (ana) divides repeatedly inside the diaphysis (d) to form metaphyseal vessels (mv) within the metaphysis (m). The mv terminate as the metaphyseal vascular capillary plexus (mvp) supplying the calcifying zone (cz, also known as the degenerating hypertrophic zone) of the metaphysis. The hypertrophic zone (hz) normally is not penetrated by the pvp or mvp, but may rarely be penetrated by transphyseal vessels (tp). Additional features include: a articular zone of epiphyseal cartilage; eb, endosteal bone; mc, medullary cavity; pb, periosteal bone; and rez, resorption zone of the metaphysis. Micro-anatomical terminology is consistent with the established anatomical nomenclature for avian bones (Beaumont, ; Lutfi, ,; Wise and Jennings, ; Howlett, , ; Hunt et al., ; Duff, ; Howlett et al., ; Thorp, , ; Ali, ; Farquharson and Jefferies, 2000).

Figure 3

(A) Normal proximal femoral head with white cap of epiphyseal cartilage (e) on the metaphysis (m) above the diaphysis (d). (B) Epiphysis remained in the socket when the femur was removed from its socket (epiphyseolysis), revealing the surface of the growth plate (p) and bacterial necrosis (n) underlying the fractured remnant of the femoral head. (C) Femoral head fractured along the metaphyseal (m) plane to reveal widespread necrosis (n) within the shaft of the diaphysis (d). (D) In normal proximal tibiae, struts of trabecular bone in the metaphysis (m) transmit structural support from the diaphysis to the growth plate (physis, p) and articular cartilage (epiphysis, e). A secondary center of ossification (s) develops in the tibial epiphyseal cartilage of broilers after (approximately) day 28. (E,F) Large zones of necrosis have destroyed the central metaphysis (nm) and bacterial sequestrate (thick arrows) can occupy the calcifying zone beneath the growth plate as well as in the secondary center of ossification (s). Histologic examinations revealed microbial foci within the metaphyseal parenchyma adjacent to necrotic voids, in agreement with previous reports (Mutalib et al., ; McNamee and Smyth, 2000). Voids undercutting the bony support structure facilitate micro-fracturing of the epiphyseal-physeal cartilage (Wideman et al., 2012).

Figure 4

(A) Clinically healthy chicks were euthanized at 2 weeks of age, eviscerated and fixed by immersion in 10% buffered formalin for histological evaluation. Care was taken throughout to avoid pulling on or twisting the legs. After 5 days of fixation, the proximal half of each femur was removed with the pelvic socket still attached. The bones were decalcified, embedding in paraffin, sectioned at 5 μM thickness, and stained with hematoxylin and eosin. In two of the eight legs evaluated, narrow osteochondrotic clefts or voids (thick arrows) were evident at the boundary between the epiphysis (e) and the growth plate (physis, p) and occasionally between cartilage columns near the calcifying zone (cz) of the metaphysis (m). (B) Magnification of the clefts (thick arrows) at the junction of the epiphysis (e) and growth plate or physis (p). A penetrating epiphyseal vessel (pev) is threatened with transection. There is no evidence of inflammation or cellular debris adjacent to or within the clefts. The pre-mortem existence of these clefts cannot be proven, but clearly this key boundary zone is structurally fragile. Osteochondrotic clefts at the epiphyseal-physeal boundary and within the physeal cartilage are considered to predispose broilers to epiphyseolysis, which in turn is considered an early or initial macroscopic manifestation of BCO (Duff, , , ; Duff and Randall, ; Thorp et al., ; Bradshaw et al., 2002). Also shown are epiphyseal vascular canals (ec) and a metaphyseal vascular plexus (mvp).

Figure 5

Diagrammatic representation of the proximal head of the femur, depicting the formation of osteochondrotic clefts at the boundary between the epiphysis (e) and the growth plate (physis, p). Ascending metaphyseal vessels (mv) penetrate through canals between long columns of calcifying cells in the metaphysis (m). The metaphyseal vascular plexus (mvp) is formed by hairpin bends in fenestrated metaphyseal capillaries that return as venules coursing through the same canal (blue arrows). Translocated bacteria spread hematogenously and can exit the bloodstream through the fenestrated endothelium at the tips of the metaphyseal vascular plexus. The extravasated bacteria may adhere directly to the cartilage matrix, they colonize osteochondrotic clefts and zones of necrosis, and they form obstructive emboli in the metaphyseal vasculature. Similar hairpin bends and fenestrated capillary epithelia have been reported for the terminal epiphyseal vascular complex (evc), as well as for the terminus of penetrating epiphyseal vessels (pev) and the penetrating vascular plexus (pvp).

Figure 6

Microphotograph of the proximal femoral head of a 38-day-old broiler with definitive femoral head necrosis (hematoxylin and eosin stain; 5 μm section). Bacterial foci are circled. Arrows indicate an infected osteochondrotic cleft at the interface between the epiphyseal cartilage and the physeal cartilage. A degenerating penetrating epiphyseal vessel (pev) is associated with the cleft.

Figure 7

Cumulative incidence of total lameness for broilers that at 1 day of age were placed on clean wood shavings floor litter (L), and either remained on litter for 8 weeks (8L) or were transferred from litter to flat wire flooring (W) for the remainder of the 8-week experiment, beginning at 3 weeks of age (3L5W), 4 weeks of age (4L4W), 5 weeks of age (5L3W), or 6 weeks of age (6L2W). All birds were pushed to grow as rapidly as possible to meet their genetic potential (23 h of light per day, high density feeds provided ad libitum, thermoneutral and well ventilated environmental conditions). The incidence of lameness remained typically low as long as the broilers remained on litter, however within 5–7 days after being transferred to wire flooring the incidence of lameness increased dramatically. Total lameness for the 3L5W, 4L4W, and 5L3W groups converged to between 59 and 64% by week 8.

Figure 8

(A) Incidence of lameness as a percentage of the total number of birds per group for broilers in three experiments (#1 through #3) that received three (Saline #1, DEX #1) or six (Saline #2, DEX #2, Saline #3, Lo DEX #3, Hi DEX #3) injections of saline or dexamethasone (DEX). (B) Incidences of “severe” tibial head lesions for the uninjected and saline- or dexamethasone-injected broilers are compared in three experiments. Lesions that obviously had damaged the growth plate per se, or that contained caseous exudates, were recorded as being “severe.” Values reflect the percentages of all legs evaluated; within each group the data for lame birds and survivors are pooled. ^a,b,cValues differed between the groups within an experiment (P < 0.05; Z-test; adapted from Wideman and Pevzner, 2012).

Figure 9

Meta-analysis of total lameness through 56 days of age for broilers in Experiments 1–5 from Lines C, B, D and G that were reared on clean wood shavings litter and fed a control diet (litter-control feed), or reared on wire flooring and fed the control diet alone (wire-control feed) or the control diet mixed probiotics beginning on d 1 (wire-probiotics). ^a,b,c Values with different superscripts within an experiment differed significantly at P < 0.05 using repeated Z-tests (SigmaPlot) to compare proportions adapted from Wideman et al. (2012).