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MUSCULOSKELETAL SYSTEM

Bone

Hydatid disease in bone is uncommon. The world literature indicates an average incidence of 1.5% in bone. Sixty percent of all osteohydatid disease occurs in the spine, pelvis and hip joint. Another 28% of cases are found in long bones (femur, tibia and humerus) and 8% in the ribs and scapula. Isolated infections occur in the calvaria and phalanges. Hydatid cysts within bone are always primary, with the larva reaching the skeletal site via a systemic artery. Occasionally, there may be associated cysts in other organs, chiefly liver or lung.

The larval hydatid develops very slowly within the medullary portion of bone and almost always manifests itself in adults rather than in children. In bone, because of the resistance of osseous tissue, the parasite is unable to follow its usual mode of development. There is a minimal response on the part of the bony tissues, and no adventitial wall forms as in other parts of the body. Instead of the single large cyst seen in other host tissues, microvesicles are produced by exogenous budding and spread throughout the spongiosa of the involved bone to create a lesion which initially molds itself to the rigid bony confines. The hydatid then develops multiple exogenous daughter cysts in the absence of any adventitial pericyst, and these insinuate themselves between the bony trabeculae, destroying them. The end result is extensive destruction of the medullary bone by a multivesicular cyst.

After destroying considerable cancellous bone and forming numerous daughter cysts, the hydatid may breach the cortex and periosteum and penetrate into adjacent soft tissues. An adventitial layer may form in the periosteum and soft tissues to surround the daughter cysts and produce a confluent multilocular cyst. The bony hydatid may penetrate through the articular cartilage of the epiphysis to invade the joint and cross the joint space to involve the opposite bone. Advanced destruction and pathological fractures almost invariably ensue.

Because the course of osseous hydatid disease is usually lengthy, symptoms may appear quite late. The clinical picture is vague, with pain occurring late in the disease; pain is usually secondary to a pathological fracture or follows rupture of the hydatid through the cortex and development of a soft tissue abscess and osteomyelitis. In the spine it is a serious condition, with a high incidence of neurological involvement and paraplegia and a relatively high mortality. Local recurrences are common at most skeletal sites because of the difficulty of eradicating the disease.

As in the case of clinical symptoms, there is a similar absence of radiological findings over a long period of time. According to the work of Bellini, there are three main stages in the development of radiographic findings. The first stage is microvesicular infiltration, in which the larvae are deposited in bone and cause destruction by mechanical pressure. As the vesicles or daughter cysts grow and enlarge, cyst-like spaces develop which are lined by thin trabeculae and produce a multilocular appearance resembling a "bunch of grapes". At this stage, there is no sharp demarcation of the lesion and no new bone formation or periosteal reaction. In the second stage, which occurs late in the disease, a secondary infection is engrafted on the hydatid process and produces an inflammatory osteitis. The prolonged irritation results in the development of sclerosis and hyperostosis in the involved bone, which gradually replaces the pre-existing multilocular appearance. The third stage relates to the formation of the ossifluent abscess which occurs after the hydatid penetrates the cortex and breaks out with exuberant growth into adjacent soft tissues. The cysts that develop in the soft tissues are spherical, may reach several inches in size, and may subsequently calcify. Soft tissue hydatids may be identified by plain radiographs, especially if calcified (Fig. 3.126), but are better evaluated by ultrasound, CT or MRI (Fig. 3.127). They may also cause complications by compression of neighboring structures such as ureters, causing obstruction; there may be overgrowth of the cyst to other organs and perforation into the spinal canal or veins, leading to dissemination. Additional stages and findings in musculoskeletal hydatid disease have been described in the past decade by von Sinner.

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Fig. 3.126 Hydatid disease involving the soft tissues of the thigh in two different South African patients. (A) Multiple large, partially calcified, spherical cysts in the upper thigh of a 20-year-old man. (B) A huge noncalcified hydatid in the soft tissues of the midthigh, posterior to the femur in a young woman. Neither patient had any other demonstrable hydatid cysts. (Courtesy of the University of Cape Town Radiology Library).

Fig. 3.127 Ultrasound scan of a 7-year-old Tunisian boy, showing a soft tissue hydatid cyst of the thigh muscles with detached parasitic membranes identified by the "serpent sign". (Courtesy of Dr. Gharbi, Tunis.

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