# Lecture 15: Medical Imaging - Musculoskeletal ## Unit 1.3: Human Gross Anatomy I - Musculoskeletal System --- ## Learning Objectives By the end of this lecture, students will be able to: 1. Describe the principles and applications of different imaging modalities for musculoskeletal structures 2. Identify normal anatomy on radiographs (X-rays) of the skeleton 3. Recognize common fracture patterns and their nomenclature 4. Interpret basic CT and MRI images of musculoskeletal structures 5. Understand the advantages and limitations of each imaging modality 6. Apply systematic approaches to image interpretation --- ## Lecture Content ### I. Overview of Musculoskeletal Imaging Modalities Multiple imaging modalities serve distinct roles in evaluating musculoskeletal pathology. Radiography, commonly called X-ray, excels at visualizing bone structure, identifying fractures, and assessing skeletal alignment, though it provides poor soft tissue detail. Computed tomography offers superior bone detail with the ability to create three-dimensional reconstructions, making it ideal for complex fractures, though it involves radiation exposure and has limited soft tissue discrimination. Magnetic resonance imaging provides exquisite soft tissue contrast, allowing visualization of cartilage, ligaments, tendons, and bone marrow, though it costs more, requires longer acquisition times, and has contraindications related to implanted devices. Ultrasound permits real-time assessment of tendons, muscles, and superficial structures without radiation, though it is operator-dependent and cannot penetrate bone. Nuclear medicine studies detect metabolic bone activity, useful for identifying infection, tumors, and occult fractures, though they provide poor anatomic detail. Selecting the appropriate modality depends on the clinical question. For most skeletal complaints, plain radiography serves as the first-line study. CT provides additional detail for complex fractures and preoperative planning. MRI is preferred for soft tissue injuries and internal joint derangement. Ultrasound offers advantages for dynamic assessment and procedural guidance. Panel A: Plain radiograph of knee joint (AP and lateral views) showing bone detail, joint space, and patella alignment. Panel B: CT scan axial cross-section in bone window showing cortical detail and trabecular architecture. Panel C: MRI sagittal T2-weighted image showing ACL, PCL, menisci as distinct structures, articular cartilage layers, and bone marrow signal. Panel D: Ultrasound longitudinal view of quadriceps tendon with fibrillar pattern visible, each modality labeled with key visible structures. --- ### II. Radiography Principles Radiography uses X-rays that pass through the body and are absorbed differentially by tissues. Dense structures such as bone absorb more radiation and appear white, termed radiopaque. Air absorbs minimal radiation and appears black, termed radiolucent. Soft tissues absorb intermediate amounts and appear in various shades of gray. Standard radiographic evaluation requires a minimum of two views taken at ninety-degree angles to each other. Common combinations include anteroposterior and lateral views for most anatomic regions. Posteroanterior and lateral views are standard for the chest and hand. Additional oblique views and specialized projections address specific joints and suspected pathology. A systematic approach to radiograph interpretation ensures thorough evaluation and reduces missed findings. The ABCDs framework provides a useful structure. Assessment of alignment examines joint surfaces for congruence and vertebral body positioning. Bone evaluation considers density, cortical integrity, and trabecular pattern. Cartilage assessment, while indirect since cartilage itself is not visible, examines joint space width as an indicator of cartilage thickness. Distribution analysis considers the pattern of involvement to suggest underlying diagnoses. Soft tissue examination identifies swelling, calcification, and foreign bodies. Panel A: A-alignment shown on spine radiograph with smooth vertebral body lines drawn along anterior, posterior, and spinolaminar surfaces. Panel B: B-bone density comparison between normal and osteoporotic specimens with cortical thinning and trabecular loss patterns labeled. Panel C: C-cartilage/joint space assessment showing normal versus narrowed joint space in knee radiograph as indirect marker of cartilage loss. Panel D: D-distribution patterns comparing rheumatoid arthritis (proximal, symmetric involvement) versus osteoarthritis (weight-bearing, asymmetric) and S-soft tissue findings including swelling and calcification examples. --- ### III. Normal Radiographic Anatomy Understanding normal radiographic appearance is essential for recognizing pathology. Long bones demonstrate several distinct regions on radiographs. The cortex appears as a dense white outer layer providing structural integrity. The medulla shows a lighter interior containing trabecular bone in a characteristic pattern. The periosteum is not visible on radiographs unless elevated by pathology such as tumor or infection. In skeletally immature patients, the epiphysis represents the end of the bone with a visible radiolucent growth plate called the physis. The metaphysis is the flared region adjacent to the growth plate. The diaphysis comprises the shaft. Joints appear as radiolucent spaces because articular cartilage does not absorb X-rays. The subchondral bone beneath the cartilage appears as a dense white line. Normal alignment shows congruent articular surfaces with uniform joint space width. Spinal radiographs demonstrate vertebral bodies as roughly rectangular structures. On anteroposterior views, the pedicles appear as paired ovoid structures resembling eyes. Disc spaces appear radiolucent since the intervertebral disc is not directly visible. Alignment is assessed by tracing smooth curves along the anterior vertebral body line, the posterior vertebral body line, and the spinolaminar line. Panel A: Long bone anatomy (femur) with cortex, medulla, epiphysis, physis, metaphysis, and diaphysis labeled with magnified insets of each region. Panel B: Joint anatomy (knee) showing joint space, subchondral bone plate, and normal alignment of congruent articular surfaces. Panel C: Spine AP view with pedicle "eyes" as paired ovoid structures and disc spaces as radiolucent intervals. Panel D: Spine lateral view with anterior, posterior, and spinolaminar alignment lines drawn in different colors for systematic assessment. --- ### IV. Fracture Nomenclature Complete fracture description requires attention to several elements. The bone involved must be identified. The location within the bone is described as proximal, middle third or shaft, or distal, and whether the fracture is intra-articular or extra-articular. The pattern describes the configuration of the fracture line. Displacement characterizes the relationship between fracture fragments. Finally, open versus closed status indicates whether the fracture communicates with the external environment through a skin wound. Fracture patterns reflect the mechanism of injury. A transverse fracture runs perpendicular to the long axis of the bone and typically results from a direct blow. An oblique fracture crosses the bone diagonally and suggests angulation or compression forces. A spiral fracture winds around the shaft in a helical pattern and indicates torsional injury. A comminuted fracture produces more than two fragments and results from high-energy trauma. A segmental fracture isolates a segment of bone between two fracture lines. An avulsion fracture occurs when a tendon or ligament pulls a fragment of bone away from its attachment. A compression fracture, most common in vertebrae, results from axial loading that crushes the bone. An impacted fracture occurs when one fragment is driven into another. In children, incomplete fractures include the greenstick fracture where one cortex breaks while the other bends, and the torus or buckle fracture where the cortex buckles under compression without complete disruption. Displacement is described in several dimensions. Translation refers to shift in position, specified as medial, lateral, anterior, or posterior. Angulation describes the direction the apex of the angulation points. Rotation indicates malrotation of the distal fragment. Shortening or distraction describes change in overall length. Percentage contact or apposition describes how much of the fracture surfaces remain in contact. Panel A: Transverse fracture (perpendicular to shaft from direct blow), oblique fracture (diagonal from angular force), and spiral fracture (helical line from torsional injury) with mechanism icons. Panel B: Comminuted fracture (multiple fragments from high-energy trauma), segmental fracture (isolated segment between two fracture lines), and avulsion fracture (tendon pulling fragment from attachment). Panel C: Compression fracture (axial loading causing vertebral wedging) and impacted fracture (fragments telescoped into each other). Panel D: Pediatric incomplete fractures including greenstick (one cortex broken, other bent) and torus/buckle (cortical bulge without complete disruption). --- ### V. Common Fractures by Region #### Upper Limb Fractures The clavicle is the most commonly fractured bone, typically breaking at the junction of the middle and lateral thirds where it is weakest. Radiographs may show displaced fragments with characteristic superior displacement of the medial fragment due to sternocleidomastoid muscle pull. Proximal humerus fractures are classified by the Neer system, which evaluates four parts: the humeral head, the shaft, the greater tuberosity, and the lesser tuberosity. Radiographic evaluation identifies which parts are displaced. At the elbow, radial head fractures represent the most common adult elbow fracture. The fat pad sign is an important indirect finding where the posterior fat pad, normally not visible, becomes displaced by effusion and appears as a lucency posterior to the distal humerus. Olecranon fractures typically occur from direct trauma or triceps avulsion. In children, supracondylar fractures predominate and require careful assessment for displacement. Distal radius fractures carry eponymous names based on displacement direction. A Colles fracture demonstrates dorsal displacement of the distal fragment, producing the classic dinner fork deformity when viewed from the side. A Smith fracture shows volar displacement, essentially a reverse Colles pattern. A Barton fracture is an intra-articular fracture involving the dorsal or volar rim of the radius. In the hand, scaphoid fractures result from falls onto an outstretched hand. The scaphoid is at risk for avascular necrosis due to its retrograde blood supply, where the proximal pole receives blood through vessels that enter distally. Initial radiographs may be negative, with the fracture becoming visible only after bone resorption occurs at the fracture site. A boxer's fracture involves the neck of the fifth metacarpal, typically from punching. Bennett's fracture is a fracture-dislocation at the first carpometacarpal joint. Panel A: Clavicle fracture at typical middle-third location with fragment displacement and sternocleidomastoid muscle force vectors causing superior displacement of medial fragment. Panel B: Proximal humerus four-part anatomy diagram with Neer classification examples, and elbow radiograph with positive posterior fat pad sign (sail sign) indicating radial head fracture. Panel C: Distal radius showing Colles fracture (dorsal displacement, dinner fork profile) versus Smith fracture (volar displacement) in lateral view. Panel D: Scaphoid fracture location in anatomical snuffbox region with retrograde blood supply diagram and avascular necrosis risk zone of proximal pole highlighted. --- #### Lower Limb Fractures Hip fractures carry important implications based on their location relative to the joint capsule. Femoral neck fractures are intracapsular and carry risk of avascular necrosis because the fracture may disrupt the blood supply to the femoral head from the medial circumflex femoral artery. Intertrochanteric fractures occur between the greater and lesser trochanters and are extracapsular with preserved femoral head blood supply. Subtrochanteric fractures occur below the lesser trochanter and typically result from high-energy trauma. At the knee, tibial plateau fractures result from axial loading with valgus or varus force, compressing the articular surface. CT is often required for complete characterization and surgical planning. Patellar fractures are most commonly transverse. The Segond fracture is an avulsion of the lateral tibial rim that serves as a specific indicator of anterior cruciate ligament injury. Ankle fractures are classified by the Weber system based on the level of the fibular fracture relative to the syndesmosis, the ligamentous connection between the tibia and fibula. Weber type A fractures occur below the syndesmosis and are typically stable. Weber type B fractures occur at the level of the syndesmosis with variable stability. Weber type C fractures occur above the syndesmosis and are unstable, requiring surgical fixation. Foot fractures include calcaneal fractures from falls, where Bohler's angle, normally twenty to forty degrees, becomes decreased or reversed. Lisfranc injuries involve disruption at the tarsometatarsal joint complex. Jones fractures occur at the metaphyseal-diaphyseal junction of the fifth metatarsal and have poor healing due to a watershed blood supply. Fifth metatarsal base avulsion fractures result from fibularis brevis muscle pull and should be distinguished from Jones fractures because they heal more reliably. Panel A: Hip region showing femoral neck fracture (intracapsular with blood supply diagram and AVN risk), intertrochanteric fracture (between trochanters, extracapsular), and subtrochanteric location. Panel B: Knee showing tibial plateau depression from axial loading and Segond fracture (lateral tibial rim avulsion) as specific indicator of ACL injury. Panel C: Ankle with Weber A (below syndesmosis, stable), B (at syndesmosis, variable), and C (above syndesmosis, unstable) classification with syndesmosis level reference. Panel D: Foot showing calcaneal fracture with Bohler's angle measurement (normal 20-40 degrees versus decreased), and fifth metatarsal comparing Jones fracture (metaphyseal-diaphyseal junction, poor healing) versus base avulsion (more proximal, better prognosis). --- #### Spine Fractures Cervical spine fractures include several classic patterns. The Jefferson fracture is a burst fracture of the C1 ring, best seen on the open-mouth odontoid view where lateral masses of C1 overhang the lateral masses of C2 bilaterally. The hangman's fracture involves bilateral fractures of the C2 pars interarticularis, causing C2 to slide forward on C3. Odontoid fractures are classified by location within the dens. Flexion teardrop fractures involve anterior vertebral body avulsion with posterior ligament disruption and are highly unstable. Thoracolumbar spine fractures include compression fractures showing anterior vertebral body wedging with loss of height, burst fractures with comminution and potential retropulsion of fragments into the spinal canal, and Chance fractures, horizontal fractures through the vertebral body and posterior elements classically associated with seatbelt injuries. Spine radiograph assessment should evaluate vertebral body height for loss indicating compression, alignment along the anterior, posterior, and spinolaminar lines, pedicle continuity which may be disrupted in burst fractures, disc space height, and prevertebral soft tissue width in the cervical region which increases with hemorrhage or edema. Panel A: Jefferson fracture on open-mouth odontoid view with lateral mass overhang of C1 on C2 marked bilaterally as burst fracture of C1 ring. Panel B: Hangman's fracture showing bilateral C2 pars interarticularis fractures with anterior displacement on lateral view, and odontoid fracture types I-III diagram. Panel C: Thoracolumbar compression fracture with anterior wedging measurement showing loss of vertebral body height. Panel D: Burst fracture with fragment retropulsion into spinal canal on CT, and Chance fracture with horizontal fracture line through entire vertebra associated with seatbelt injuries. --- ### VI. Computed Tomography Computed tomography acquires multiple X-ray projections around the patient that are reconstructed by computer algorithms into cross-sectional images. These axial source images can be reformatted into coronal, sagittal, and oblique planes, as well as three-dimensional surface renderings. CT provides excellent bone detail superior to plain radiography. It excels at characterizing complex fractures, determining the extent of articular surface involvement, and planning surgical approaches. Rapid acquisition makes it valuable in trauma settings. Different display settings called windows optimize visualization of different tissues; bone windows maximize cortical detail while soft tissue windows better demonstrate muscle and hematoma. Common musculoskeletal CT applications include complex fracture assessment, particularly acetabular fractures, calcaneal fractures, and spine fractures. Pre-operative planning benefits from three-dimensional reconstructions that demonstrate spatial relationships. Assessment of fragment position guides reduction strategies. Post-operative evaluation confirms hardware position. Panel A: Acetabular fracture on CT axial slice with fracture line through posterior column and coronal reformation showing column involvement. Panel B: Three-dimensional surface rendering of acetabular fracture showing overall pattern with rotation capability indicated for surgical planning. Panel C: Calcaneal fracture on axial and coronal CT views with joint depression visible, and cervical spine fracture on sagittal and axial views demonstrating canal compromise. Panel D: Bone window versus soft tissue window comparison of same image showing how different window settings optimize visualization of different tissue types. --- ### VII. Magnetic Resonance Imaging Magnetic resonance imaging uses strong magnetic fields and radiofrequency pulses to detect signals from hydrogen protons in tissues. Different pulse sequences produce images with varying tissue contrast without using ionizing radiation. MRI provides superior soft tissue contrast compared to any other imaging modality. Understanding basic MRI sequences aids interpretation. T1-weighted images show fat as bright signal and fluid as dark, providing excellent anatomic detail. T2-weighted images show fluid as bright and fat as intermediate signal, highlighting pathologic processes involving edema or effusion. STIR and fat-saturated sequences suppress the signal from fat, making fluid even more conspicuous for detecting bone marrow edema. Proton density sequences optimize cartilage and meniscal visualization. In musculoskeletal imaging, MRI excels at evaluating ligament integrity, where tears appear as discontinuity or abnormal signal within the normally dark ligament. Meniscal tears demonstrate linear signal extending to the articular surface. Cartilage damage appears as surface irregularity, fissuring, or frank defects. Bone marrow edema, indicating trabecular microfractures or contusions, appears bright on STIR or T2 sequences. Muscle strains demonstrate edema patterns within the affected muscle. Tendon tears show gaps, retraction, or abnormal signal replacing normal low-signal tendon. MRI contraindications include certain pacemakers and implanted devices, though many modern devices are MRI-compatible. Metallic foreign bodies near vital structures such as the eye pose risks. Claustrophobia may limit examination completion. Panel A: Sagittal MRI view with normal ACL, PCL, and menisci labeled showing dark signal on T2-weighted imaging as reference for normal anatomy. Panel B: ACL tear with discontinuity and surrounding edema compared to normal, and meniscal tear with linear signal reaching articular surface in magnified view. Panel C: STIR sequence showing bright bone marrow edema in femoral condyle (bone bruise) adjacent to normal dark marrow signal. Panel D: Full-thickness cartilage defect with exposed subchondral bone demonstrating MRI superiority for soft tissue assessment. --- ### VIII. Ultrasound Ultrasound imaging uses high-frequency sound waves to create real-time images. Sound waves reflect at tissue interfaces, creating images based on acoustic impedance differences. The technique involves no ionizing radiation and provides dynamic assessment capability. Ultrasound advantages include portability allowing bedside examination, real-time imaging for dynamic assessment of structures during movement, and the ability to guide needles during procedures. It is cost-effective and widely available. Pediatric hip evaluation for developmental dysplasia relies on ultrasound because the largely cartilaginous infant hip is not well visualized on radiographs. Musculoskeletal applications include rotator cuff evaluation where tears appear as discontinuity or non-visualization of the tendon. Tendon pathology throughout the body, including Achilles tendinopathy or tear, tennis elbow, and flexor tendon injuries, benefits from ultrasound assessment. Muscle tears demonstrate disruption of the normal fibrillar pattern with hematoma formation. Joint effusions appear as anechoic fluid collections. Ultrasound guidance improves accuracy and safety of joint injections and aspirations. Limitations include operator dependence requiring significant skill for acquisition and interpretation. Sound waves cannot penetrate bone, creating acoustic shadowing that limits visualization of structures deep to bone. Deeper structures may be difficult to assess. Body habitus affects image quality in obese patients. Panel A: Rotator cuff showing normal supraspinatus tendon fibrillar pattern versus full-thickness tear with retraction and fluid-filled gap. Panel B: Achilles tendon longitudinal view showing normal tendon versus ruptured tendon with gap measurement indicated. Panel C: Joint effusion demonstrating anechoic fluid distending joint capsule with comparison to normal joint. Panel D: Guided injection technique showing needle (hyperechoic line) entering joint space with real-time positioning capability for improved accuracy and safety. --- ### IX. Nuclear Medicine Bone scintigraphy, commonly called a bone scan, uses technetium-99m labeled bisphosphonate compounds that accumulate in areas of increased osteoblastic activity. The radiotracer distributes throughout the skeleton, with areas of increased metabolic activity appearing as hot spots of increased tracer uptake. Bone scan applications include detection of occult fractures such as stress fractures that may not be visible on initial radiographs, screening for metastatic disease where multiple scattered hot spots suggest disseminated malignancy, evaluation for osteomyelitis where infection causes increased uptake, early detection of avascular necrosis which initially shows decreased uptake before later showing increased activity during repair, and characterization of metabolic bone disease such as Paget's disease. The three-phase bone scan provides additional information. The flow phase shows arterial perfusion. The blood pool phase demonstrates soft tissue hyperemia. The delayed phase acquired several hours after injection reflects bone turnover and provides the standard bone scan image. PET-CT using fluorodeoxyglucose provides metabolic imaging with better anatomic correlation than standard bone scan. It is primarily used for oncologic staging and can identify metabolically active lesions with precise anatomic localization through the fused CT images. Panel A: Whole-body bone scan showing normal biodistribution with kidneys and bladder visible from tracer excretion as baseline reference. Panel B: Comparison scan showing multiple scattered hot spots indicating metastatic disease, and focal uptake in tibial shaft indicating stress fracture location. Panel C: Three-phase bone scan demonstration showing flow phase, blood pool phase, and delayed phase with corresponding interpretive information for each. Panel D: PET-CT fusion image showing FDG-avid lesion with CT providing precise anatomic localization through fused imaging. --- ### X. Systematic Approach to Image Interpretation A systematic approach to musculoskeletal image interpretation reduces errors and missed findings. The process begins with confirming patient information including name, date, and relevant clinical history. Technical assessment ensures adequate image quality and appropriate positioning with all expected views obtained. Systematic review examines all bones, joints, and soft tissues visible on the images without focusing exclusively on the area of primary concern. Comparison with prior studies when available helps identify interval changes. Clinical correlation integrates imaging findings with the patient's history and examination. Common interpretive pitfalls include satisfaction of search, where the interpreter stops looking after finding one abnormality and misses additional pathology. Edge-of-image pathology is easily overlooked when attention focuses on the image center. Subtle injuries at certain locations, particularly scaphoid fractures and cervical spine injuries, may be missed on initial evaluation. In children, normal growth plates can be mistaken for fractures, and fractures through physes may be subtle. Accessory ossicles, which are normal anatomic variants, may be confused with acute avulsion fractures. Key normal variants that should not be mistaken for pathology include accessory ossicles such as the os acromiale at the shoulder and os peroneum at the foot. A bipartite patella is a congenital variant with a separate superolateral patellar fragment. Nutrient foramina are oblique channels through the cortex with smooth corticated margins, distinct from fracture lines. Sesamoid bones are normal ossicles within tendons. Panel A: Satisfaction of search example showing radiograph with obvious distal radius fracture and subtle scaphoid fracture that might be missed if examiner stops looking after first finding. Panel B: Edge-of-image pathology showing calcaneal fracture at film edge easily overlooked when attention focuses on image center. Panel C: Normal variants comparison showing accessory ossicle (smooth, corticated margins) versus acute avulsion fragment (irregular margins, soft tissue swelling), and nutrient foramen (oblique, corticated channel) versus fracture line (irregular, non-corticated). Panel D: Bipartite patella example with characteristic superolateral fragment location as normal developmental variant not to be confused with fracture. --- ### XI. Clinical Correlations Imaging appropriateness requires balancing diagnostic benefit against radiation exposure following the ALARA principle: As Low As Reasonably Achievable. Clinical decision rules, such as the Ottawa ankle rules and Canadian C-spine rules, provide evidence-based guidance for when imaging is necessary, reducing unnecessary studies while maintaining sensitivity for significant injuries. Fracture healing progresses through predictable phases visible on imaging. The inflammatory phase during the first week shows soft tissue swelling on radiographs. The reparative phase over weeks two through six demonstrates callus formation as hazy mineralization bridging the fracture. The remodeling phase over months to years shows callus remodeling with gradual restoration of normal cortical architecture. Complications detectable on imaging include malunion, where the fracture heals in an abnormal position visible as angular or rotational deformity. Nonunion represents failed healing with a persistent gap between fragments and sclerotic, rounded fracture edges. Delayed union shows slower than expected healing. Avascular necrosis demonstrates loss of blood supply with eventual collapse, particularly concerning in femoral neck and scaphoid fractures. Osteomyelitis shows bone destruction with periosteal reaction. Panel A: Normal healing progression at week one (soft tissue swelling only) and week four (early callus formation as hazy mineralization). Panel B: Continued healing at week eight (bridging callus) and six months (remodeled cortex with restored architecture). Panel C: Malunion showing healed femur with residual angular deformity, and nonunion with persistent fracture gap, sclerotic rounded edges, and hardware failure. Panel D: Avascular necrosis of femoral head showing progressive collapse with subchondral lucency (crescent sign) and eventual femoral head deformity. --- ## Summary - Plain radiography serves as the first-line imaging study for bone pathology and requires a minimum of two views at ninety-degree angles - The systematic ABCDs approach ensures complete radiograph evaluation: Alignment, Bone, Cartilage, Distribution, and Soft tissues - Fracture descriptions include the bone involved, location, pattern, displacement, and whether open or closed - CT provides excellent bone detail with three-dimensional reconstruction capability for complex fracture characterization - MRI offers superior soft tissue contrast for evaluating ligaments, tendons, cartilage, and bone marrow - Ultrasound enables real-time, dynamic assessment of tendons and muscles without radiation exposure - Selecting the appropriate imaging modality based on the clinical question optimizes diagnostic yield while minimizing unnecessary radiation --- ## Key Terms | Term | Definition | |------|------------| | Radiopaque | Appears white on radiograph because the structure absorbs X-ray radiation | | Radiolucent | Appears dark on radiograph because the structure allows X-rays to pass through | | Comminuted | Fracture pattern with more than two fragments resulting from high-energy injury | | STIR | MRI sequence with fat suppression that shows edema as bright signal | | Bone scan | Nuclear medicine study using technetium-labeled bisphosphonate to detect areas of increased osteoblastic activity | | ALARA | As Low As Reasonably Achievable; principle guiding radiation dose minimization | --- *This content is subject to the [MIT License](https://opensource.org/licenses/MIT). © 2024–2026 Hibbert School of Medicine.*