Department of Radiodiagnosis and Imaging ASCOMS, Jammu
-Infection of bone and marrow is known as osteomyelitis.-Osteomyelitis is divided into:Acute osteomyelitisSubacute osteomyelitisChronic osteomyelitis There are certain types of named osteomyelitis;Brodies abscessChronic multifocal osteomyelitisSclerosing osteomyelitis of Garré
- Three basic mechanisms allow an infection to reach the bone;1. Haematogenous spread2. Contigious source of infection3. Direct implantation
- Primary haematogenous osteomyelitis is characterized by an acute infection of bone caused by seeding of bacteria within the bone from a remote source- Haematogenous osteomyelitis usually occurs during period of growth and thus occur primarily in children. However, all ages may be affected and cases are even found in old age.
Type of organisms Bacteria, viruses and fungi can all infect bone, soft tissues and joints. The pattern of infection depends not only on the microorganism but also on the route of infection and the patients resistance or immunity. In general, bacterial infections are more destructive and move rapidly. Fungi and atypical organisms and tend to produce slow and chronic infections with an infiltrative pattern that may mimic malignancy. Tuberculosis and brucellosis exhibit a variety of patterns that range from aggressive to indolent and reparative
Pathogens causing haematogenous musculoskeletal infection are generally those associated with primary bacteraemias. Of these, Staphylococcus aureus is by far the most important, with Haemophilus influenzae (in the unimmunized), Streptococcus pneumoniae, the beta-haemolytic streptococci and the aerobic Gram- negative rods also playing a role.
Pathophysiology:- Metaphysis of the long bones are highly vascularized zones. From the diaphysis the medullary arteries reach upto the growth plate—the area of greatest activity and branch into capillaries. The venous systems in this area drains towards diaphysis
Thus, the vessel in this zone are arranged in the form of loop (hair pin arrangement) resulting in ―sluggish flow‖ of blood, leading to bacterial enlodgement and thus haematogenous osteomyelitis.
- The degree of involvement of bone varies in infants, children and adults being related to vascular difference of bone at these ages.- In infants, vessels penetrate the epiphyseal plate. Metaphyseal infection can thus pass to the epiphysis and then the joint.
- Acute pyogenic arthritis is therefore a relatively common sequelae of osteomyelitis in infants.- The periosteum in infants is very loosely attached to underlying bone pus easily elevates the periosteum and so can extend to the epiphyseal plate along the shaft.
- In childhood, between 2 and 16 years, the epiphyseal plate blocks extension of the infection.- The epiphysis and joints are thus less frequently affected.
In adults, after epiphyseal plates are fused, metaphyseal and epiphyseal vessels are again connected so that septic arthritis can recess. Periosteum, however, is well bound down and articular infections via metaphyseal route are less likely.
The infection spreads laterally, into the subperiosteal space or to the joints in which synovial reflections extend to the metaphysis, such as shoulder, elbow and hip joints.
- The formation of pus in the bone deprives local cortex and medulla of its blood supply.- Dead bone is formed as a result is resorbed by granulation tissue. However pieces of dead bone, especially if cortical or surrounded by pus, are not resorbed and remain as ―sequestra‖
As sequestra are devitalised they remain denser than surrounding vital bone, which becomes demineralised due to hyperaemia and immobilisation.
Reparative new bone formation starts roughly 10 days after the onset of infection and new bone is generally laid down along the periosteum resulting in formation of ―involucrum‖. As periosteum is poorly attached in infants, involucrum formation is greater and so is the resorption of dead bone and healing.
The hallmark of infection is that aggressive and rapidly changing features (lysis, cortical breach and fracture) are mixed with slower reactions (sclerosis, heterotopic bone and periosteal reaction). If an abscess cavity forms in bone and breaks through to the soft tissues and skin the discharging sinus is a pathognomonic sign of infection. The hole in the bone develops a sclerotic margin and is called a cloaca.
DIAGNOSIS Early diagnosis of acute osteomyelitis is critical because prompt antibiotic therapy may prevent necrosis of bone. Osteomyelitis is primarily a clinical diagnosis, although the clinical picture may be confusing. An inadequate or late diagnosis significantly diminishes the cure rate and increases the degree of complications and morbidity; for these reasons, imaging modalities are essential to confirm the presumed clinical diagnosis and to provide information regarding the exact site and extent of the infectious process.
Clinical features :- Haematogenous osteomyelitis usually presents with a slow insidious progression of symptoms.- Direct osteomyelitis generally is more localised, with prominent signs and symptoms.
- General symptoms of osteomyelitis are :- Fever Fatigue Irritability Malaise Restriction of movement of limb Local edema, erythema and tenderness
Lab Findings:- WBC counts are elevated with increased polymorphoneuclear leukocyte count.- C-relative proteins – level is elevated and is more useful than ESR.- ESR is usually elevate 90%.- With osteomyelitis, culture or aspiration findings in samples of infected site are important with over 25% cases pus being sterile.- Blood culture results are positive in 50% patients with haematogenous osteomyelitis.
Radiological Findings The evaluation usually begins with plain radiographs in all patients suspected of having osteomyelitis; plain radiographs maysuggest the correct diagnosis,exclude other diagnostic possibilities, orprovide clues for underlying pathologic conditions.
Radiography;- The earliest radiographic signs of bone infection are soft tissue swelling and loss of fascial planes. These are usually encountered with in 24 to 48 hours of infection.
Typical early bony changes include: lytic lesions, periosteal thickening endosteal scalloping, osteopenia, loss of trabecular architecture, and new bone apposition- The destructive lytic lesion, usually occurs within 7 to 10 days .- This is followed by elevation of periosteum and layered new bone formation after 3 to 6 weeks.- The dead bone (i.e. sequestrum formation) occurs at 3-8 weeks. It appears dense as it does not participate in normal bone mineral metabolism because of loss of its vascular supply.
- Osteopenia in the surrounding bone due to hyperaemia, enhances the density of the sequestrum.- With adequate treatment in infants and children, remodelling reverts the appearance of bone to normal with normal growth, unless the epiphyseal plate/ epiphysis have been damaged. However, in adults the affected bone often remains sclerotic and irregular in outline.
Ultrasound Ultrasound (US) has multiple advantages: it is readily accessible, can be performed quickly without delay and with minimal discomfort to the patient, it is useful in regions that are complicated by orthopedic instrumentation and therefore might not be well seen with MRI or CT, is useful in patients in whom MRI is contraindicated, has a lower cost, does not use ionizing radiation, and offers real time imaging.
US can detect features of osteomyelitis several days earlier than can conventional radiographs (predominately in children). Acute osteomyelitis is recognized by elevation of the periosteum by a hypoechoic layer of purulent material. Power Doppler sonography is useful to highlight hyperemia around the periosteum and surrounding soft tissue abscesses
Radionuclide scanning- Where plain radiographs cannot show changes in osteomyelitis before 10-14 days, bone scintigraphy shows positive findings much earlier, usually within 3 days and sometimes as early as 24-48 hours.- Standard technique involves ―triple phase‖ bone scanning using ―technetium 99 agents).
- The first phase shows blood flow, second phase is the blood pool phase of the inflamed region and the third phase which is the delayed skeletal scintigraphy phase involves acquiring images after 2-4 hours and represents uptake by bone.- Osteomyelitis is hot on all phases.
A false positive result can occur in;degenerative disease,healing fractureloose prosthesiscrystal arthropathiesArthritisNeoplasiacellulites
CT scanning CT provides excellent multiplanar reconstructions of the axial images allowing delineation of even the most subtle osseous changes useful method to detect early osseous erosion and to document the presence of sequestrum, foreign body, or gas formation Though of less value in diagnosis, CT demonstrates changes in subacute or chronic osteomyelitis well. Sequestra, as on conventional films is shown as area of dense or high attenuation spicules of bone lying in areas of osteolysis.
- Cloacae, periostitis and local soft tissue masses are shown and enhance on I.V. contrast administration.- CT guided biopsy is used to obtain material for culture.
M.R.I. findings MRI is highly sensitive for detecting osteomyelitis as early as 3 to 5 days after the onset of infection MRI demonstrated osteomyelitis as early as isotope scanning and where available is the modality of choice in diagnosis of musculoskeletal infections. commonly used sequences include T1, T2 and STIR . The combination of short-tau inversion-recovery (STIR) and T1 spin echo sequences shows a high sensitivity and specificity
using appropriate sequences, changes in bone and soft tissue oedema may be identified early, as well as ischemia and destruction of cortex or marrow. Fatty marrow (medulla) is bright in signal on T1WI, while compact cortex, having less fluid, has a low signal. Oedema and inflammation increase signal dramatically on T2- weighted and especially on STIR.
soft tissue extension of pus through cloacae and para- osseous abscesses may be seen. Areas that become devitalized or necrotic shows loss of signal and will not enhance after I.V. gadolinium. On MRI, a sequestrum is seen as a low signal intensity structure on T1-weighted and STIR sequences, whereas the surrounding granulation tissue is intermediate to low signal intensity on T1-weighted images and high signal intensity with STIR or T2-weighted sequences.
With use of intravenous contrast (gadolinium), the granulation tissue is enhanced, whereas the sequestrum remains low Periosteal reaction and cortical bone are separated by linear intermediate to high signal intensity on T2- weighted or STIR images signal intensity Overall, MRI has been shown to be more sensitive and specific than isotope bone scan in detection, localisation and differentiation of osteomyelitis, cellulitis and abscess
Subacute osteomyelitis Those who have suffered from bone infection for several days will start to exhibit bone reaction and destruction. On plain radiographs a new periosteal reaction with a fluffy margin is typical. US will show increasing soft tissue oedema and subperiosteal fluid. MR is now probably the most sensitive technique as the marrow changes predominate. CT and US have limited roles.
On MR a ‘penumbra’ sign has been described on T1 spin-echo images. This probably represents a layer of granulation tissue. It is strongly suggestive of osteomyelitis
Chronic Osteomyelitis- Chronic bone infections usually result from inadequately treated acute osteomyelitis or from infections following compound bone fractures.
Pathology- Acute osteomyelitis commonly leads to chronic osteomyelitis because of one of following reasons:- Delayed or inadequate treatment—delay causes spread of pus within the medullary cavity subperiosteally. This results in destruction of cancellous bone leading to formation of cavities and sequestra are responsible for persistent infection.
- Type and virulence of organism – sometime despite early adequate treatment of acute osteomyelitis the body defence mechanism may not be able to control the damaging influence of highly virulent organism and infection persists. Eg. H. Influenzea, enterobacter and pseudomonas species.- Reduced host resistance—malnutrition and underlying disease conditions compromise the body’s defence mechanism, thus letting the infection persist.
Disease states known to predispose patients to osteomyelitis include- Diabetes- sickle cell disease,- AIDS- I.V.- drug abuse,- alcoholism,- chronic steroid use,- immunosuppression and- chronic joint disease.
When the infection persist because of the above reasons, the host bone responds by generating more and more sub- periosteal new bone formation. This results in thickening of the bone. The sub periosteal bone is deposited in a very irregular fashion so that the osteomyelitis bone has an irregular surface. The continuous discharge of pus results in the formation of a sinus. With time, the wall of the sinus gets fibrosed and the sinus becomes fixed to the bone.
Clinical Features in chronicOsteomylitis- Pain.- Local swelling.- Chronic discharging sinus.- Thickened, irregular bone- which can be appreciated on comparing the girth of the affected bone with that of bone on normal side. Adjacent joint stiffness – which may be either, due to excessive soft tissue scarring or because of associated arthritis of the joint.
Radiological features Radiographs show Thickning and irregularities of cortex There are sclerotic and leucent areas admixed with bony thickning and defomities . This appearance is because of granulation tissue seen surrounding the sequestrum. Involucrum and cloacae are also seen .
Thickening of cortex, reactive sclerosis, destruction of medullarycavity, fistula
Radiographic signs of activity in chronicosteomyelitis Change from previous radiographs Poorly defined areas of osteolysis Thin linear periostitis Sequestration
SINOGRAPHY Opacification of a sinus tract can produce important information that influences the choice of therapy. In this technique, a small flexible catheter is placed within a cutaneous opening. Retrograde injection of contrast material defines the course and extent of the sinus tract and its possible communications with neighboring structures. Sinography may be combined with CT for better delineation of the sinus tracts.
Ultasound In chronic osteomyelitis, US can also be used to assess involvement of the adjacent soft tissues. Soft tissue abscesses related to chronic osteomyelitis are identified as hypoechoic or anechoic fluid collections, which may extend around the bony contours
CT In chronic osteomyelitis, CT demonstrates abnormal thickening of the affected cortical bone, with sclerotic changes, encroachment of the medullary cavity, and chronic draining sinus. Although CT may show these changes earlier than do plain radiographs, CT is less desirable than MRI because of decreased soft tissue contrast as well as exposure to ionizing radiation.
The major role of CT in osteomyelitis is the detection of sequestra in cases of chronic osteomyelitis, as these pieces of necrotic bone can be masked by the surrounding osseous abnormalities on conventional radiography. The presence of pieces of sequestered bone suggests activity of the infectious process, and their detection is helpful to guide the therapeutic options. CT is superior toMRI for the detection of sequestra, cloacas, involucra, or intraosseous gas and can help in the guidance of needle biopsies and joint aspiration
MRI MR has been advocated as the imaging modality to distinguish regions of active infection from fibrotic regions representing scars from previous infection . Active foci of infection tend to have low to intermediate signal on T 1WI and high signal intensity on T2WI which is in marked contrast to the low signal of thickened surrounding bone.
Named types of Osteomyelitis Brodies abscess Chronic multifocal osteomyelitis Sclerosing osteomyelitis of Garré
Brodies abscess Described by Brodie in 1832. This is a description given to an intraosseous abscess that is surrounded by intense sclerosis . It is probably one type of subacute infection. Brodie’s abscesses are especially common in children, more typically boys. In this age group, they appear in the metaphyses, particularly that of the distal or proximal portions of the tibia. As a rule sequestra are absent and a radioleucent tract may be seen extending from the lesion into the growth plate(tunneling). The plain radiographs may mimic osteoid osteoma but cross- sectional imaging will demonstrate a substantial cavity and biopsy will reveal infection
Chronic multifocal osteomyelitis In the 1970s it was noted that a number of children presented with a low-grade form of bone disease that behaved clinically like an acute osteomyelitis. Typically it affected the long bones and went on to a sclerotic reaction. The first episode would settle and some months or even a few years later there would be recurrence at another site. No organisms are grown and the course of the disease becomes chronic and relapsing.
The clinical importance is to avoid repeated biopsy once the relapsing nature of the condition has been recognized. Plain radiographs are essential to recognize the bone infection. Skeletal scintigraphy is a good method of screening for other lesions whilst MRI is the best means of judging extent and activity
Sclerosing osteomyelitis of Garré A rare type of osteomyelitis occuring in children and young adults presenting with insidious onset of pain, pyrexia and swelling. Symptoms recur at intervals for several years and subside graudlly . . Radiological appearance is of intense sclerosis resulting in thickned bone. There is predilection for involvement of mandible and shaft of long bones.
There is no discharging sinus No necrosis No purulent exudate Little granulation tissue
Non Pyogenic bone infections Tuberculosis Fungi Syphilis
Tuberculosis Tuberculous bone infection occurs secondarily as a resolt of hematogenous spread from a primary source such as lung or genitourinary tract. Bone infection is most typically slow growing and indolent. Tuberculous ‘caries’ is seen where the margin of the bone is scalloped and eaten away. Large ‘cold’ abscesses occur. This means that the patient is surprisingly well given the size of the collection
Little or no surrounding reactive bone with presence of osteopenia Affects epiphysis, metaphysis and diaphysis. Eccentric area of osteolysis is seen in metaphysic Transepiphyseal spread of lytic lesion No sequestrum formation is seen. Occasionally, destruction in the mid diaphysis of a short tubular bone of the hand or foot(tuberculous dactylitis) may produce a fusiform enlargement of the entire diaphysis is called as spina vetosa.
Lytic lesion with soft tissue swelling No sclerosis or periosteal new bone formation
Fungi A variety of fungi may affect bones and joints. In general the infections tend to be slowly developing and difficult to eradicate; they may mimic tumours. There is an increased incidence in areas of the world where the organisms naturally reside. Fungal infections in the musculoskeletal system are seen more often in immunosuppressed patients. Fungi should be considered when the bacteriological findings do not fit the clinical presentation.
Destructive lesion , minimal sclerosis and no periosteal reaction
Syphilis Skeletal presentations are varied and mimic many other diseases Congenital syphilis may cause a symmetric periostitis with lamination. Granulomas occurring in the metaphyses of long bones produce lytic areas known as Wimbergers sign The growth plate may be abnormal with lytic bands in the adjacent bone
Complications AND Sequale Growth disturbance (vicinity of growth plate) Pathological fractures Sinus tract neoplasm