MCQs – Basic 4

Modern Study Review (AI-Generated)

High-Yield Summary

Gait biomechanics and bone physiology are cornerstone topics in orthopaedic exams, emphasizing ground reaction forces, muscle activation patterns, and bone microanatomy. This topic is a staple of the Royal College exam, focusing heavily on the spatial relationship of ground reaction forces during gait phases and the biochemical composition of bone. The single most important trade-off in clinical scenarios is understanding how subtalar joint position (everted vs inverted) dictates midfoot mobility versus rigidity, directly impacting foot function during stance and push-off. While classic exam answers emphasize static joint moments, modern practice increasingly integrates dynamic muscle activation and energy efficiency principles in gait analysis.

High-Yield Decision Matrix

Active Recall Q&A

Ground Reaction Force & Gait Mechanics

Q: Where is the ground reaction force located at initial contact relative to the ankle, knee, and hip?
A: Anterior to the ankle, anterior to the knee, and anterior to the hip.
Related Pearl: This anterior positioning creates flexion moments at the hip and ankle, requiring active muscular stabilization to prevent collapse.

Q: During loading response, where is the ground reaction force relative to the ankle, knee, and hip?
A: Posterior to the ankle, posterior to the knee, and anterior to the hip.
Related Pearl: Posterior forces at ankle and knee generate extension moments that aid in weight acceptance and shock absorption.

Q: At midstance, where is the ground reaction force located relative to the ankle, knee, and hip?
A: Anterior to the ankle, anterior to the knee, and posterior to the hip.
Related Pearl: The posterior hip force induces hip extension, stabilizing the pelvis during single limb support.

Q: Where is the ground reaction force during terminal stance relative to the ankle, knee, and hip?
A: Anterior to the ankle, anterior to the knee, and posterior to the hip.
Related Pearl: This supports forward propulsion via ankle plantarflexion and hip extension moments.

Q: During preswing, where is the ground reaction force relative to the ankle, knee, and hip?
A: Anterior to the ankle, posterior to the knee, and posterior to the hip.
Related Pearl: Posterior knee force facilitates knee flexion, enabling limb advancement.

Muscle Activity & Function

Q: When are the hamstrings most active during the gait cycle?
A: During the swing phase.
Related Pearl: Hamstrings eccentrically decelerate the thigh to prepare for heel strike, preventing knee hyperextension.

Q: What is the primary function of the hamstrings in late swing phase?
A: To decelerate the thigh.
Related Pearl: This eccentric control protects the knee joint and coordinates smooth limb advancement.

Center of Gravity & Motion

Q: What is the approximate vertical displacement of the center of gravity during walking?
A: About 2.5 cm (1 inch) up and down; increases with faster walking.
Related Pearl: Minimizing vertical displacement improves energy efficiency and reduces fatigue.

Q: When is the center of gravity highest during the gait cycle?
A: At midstance.
Related Pearl: Corresponds to single limb support with maximal hip extension and stability.

Q: When is the center of gravity lowest during the gait cycle?
A: During loading response when all joints are flexed.
Related Pearl: Lowering the center of mass helps absorb impact forces during weight acceptance.

Subtalar Joint & Foot Mechanics

Q: What motion does the subtalar joint perform at heel strike?
A: Eversion, which unlocks the midfoot.
Related Pearl: Unlocking allows shock absorption and foot adaptability on uneven terrain.

Q: What is the role of the tibialis posterior tendon during mid to terminal stance?
A: It inverts the subtalar joint to lock the midfoot for push-off.
Related Pearl: Midfoot locking creates a rigid lever essential for efficient propulsion.

Q: How are the axes of the calcaneocuboid and talonavicular joints aligned when the subtalar joint is everted?
A: They are aligned, allowing midfoot motion and foot accommodation to loading.
Related Pearl: This alignment facilitates foot flexibility during stance phase.

Q: What happens to the axes of the calcaneocuboid and talonavicular joints when the subtalar joint is inverted?
A: They become divergent, locking the midfoot.
Related Pearl: Locked midfoot is critical for stable push-off and energy transfer.

Hip Joint Moments

Q: What hip moment exists during heelstrike and why?
A: A flexion moment because the joint reaction force passes anterior to the hip.
Related Pearl: Hip extensors must activate to counteract this and stabilize the pelvis.

Q: How does the hip joint reaction force change at midstance?
A: It moves posterior to the hip, creating an extension moment.
Related Pearl: This extension moment stabilizes the hip during single limb support.

Q: What hip moment is present throughout the swing phase?
A: A flexion moment.
Related Pearl: Hip flexors are active to advance the limb forward.

Muscle Activation at Heelstrike

Q: Which muscles are active at heelstrike?
A: Tibialis anterior, extensor digitorum longus (EDL), hamstrings, and quadriceps; hip flexors are inactive.
Related Pearl: Tibialis anterior controls foot drop; hamstrings decelerate limb to prepare for stance.

Energy Consumption

Q: How much does energy consumption increase during 3-point crutch walking?
A: Approximately 50% (1.5 times normal).
Related Pearl: Increased energy demand necessitates focused gait training and assistive device optimization.

Bone Composition & Structure

Q: What percentage of bone is mineral/inorganic material and what is its main component?
A: 70%, mostly hydroxyapatite.
Related Pearl: Hydroxyapatite provides compressive strength critical for load-bearing.

Q: What percentage of bone is organic matrix (osteoid), and what does it mainly consist of?
A: 22-25%, with 98% collagen and other proteins, and 2% cells.
Related Pearl: Collagen confers tensile strength and flexibility to bone.

Q: What is the water content of bone?
A: 5-8%.
Related Pearl: Water contributes to bone’s viscoelastic and shock-absorbing properties.

Q: What percentage of the organic matrix of bone is type I collagen?
A: Approximately 90% (some sources say 95%).
Related Pearl: Type I collagen is essential for bone’s structural integrity and resistance to tensile forces.

Q: Is bone isotropic or anisotropic?
A: Bone is anisotropic.
Related Pearl: Mechanical properties vary with loading direction, influencing fracture patterns and implant design.

Mineralization & Enzymatic Activity

Q: What enzymes do matrix vesicles in bone contain and what is their function?
A: Alkaline phosphatase and pyrophosphatase; they remove calcification inhibitors and provide phosphate ions for mineral precipitation.
Related Pearl: Pyrophosphatase degrades pyrophosphate, a potent mineralization inhibitor, enhancing bone formation.

Q: Where does mineralization occur in bone?
A: In the gaps and pores within collagen fibers.
Related Pearl: This microenvironment facilitates hydroxyapatite crystal nucleation and growth.

Q: What is the function of alkaline phosphatase in bone?
A: It provides phosphate ions and renders the matrix calcifiable, playing a key role in mineralization.
Related Pearl: Deficiency causes hypophosphatasia, characterized by defective bone mineralization.

Q: Which phosphatase do osteoblasts and osteocytes have, and which do osteoclasts have?
A: Osteoblasts/cytes have alkaline phosphatase; osteoclasts have acid phosphatase.
Related Pearl: Acid phosphatase is critical for bone resorption during remodeling.

Q: Where do osteoclasts attach to bone, and what is the role of the clear zone and ruffled border?
A: Osteoclasts attach at the clear zone, which seals the resorption area; the ruffled border secretes acid to degrade bone.
Related Pearl: Carbonic anhydrase in osteoclasts produces acid essential for dissolving mineralized matrix.

Microanatomy & Vascularization

Q: What is the diameter of an osteon?
A: Approximately 250 microns.
Related Pearl: Osteon size limits nutrient diffusion and influences remodeling rates.

Q: What is the function of Volkmann canals?
A: They connect Haversian canals.
Related Pearl: Volkmann canals provide transverse vascular channels, ensuring cortical bone viability.

Q: How is blood flow distributed in diaphyseal cortical bone?
A: Inner two-thirds supplied intraosseously (centrifugal flow); outer one-third supplied by periosteum (centripetal flow).
Related Pearl: Dual blood supply supports cortical bone repair and metabolic demands.

Q: Where do cortical capillaries drain?
A: Into intramedullary venous sinusoids inside the bone (centripetal flow).
Related Pearl: Efficient venous drainage is essential for waste removal and bone homeostasis.

Fracture Mechanics & Cartilage Biology

Q: What is the average angle of a spiral fracture under experimental conditions?
A: Approximately 30 degrees.
Related Pearl: Spiral fractures result from torsional forces; angle measurement aids in injury mechanism analysis.

Q: What biochemical changes occur in the hypertrophic cartilage zone?
A: Glycogen is depleted; chondrocytes synthesize alkaline phosphatase (alk phos ?, glycogen ?).
Related Pearl: Increased alkaline phosphatase marks chondrocyte maturation and matrix mineralization during endochondral ossification.

Classic Clinical Notes

• Ground reaction force
• Initial contact anterior to ankle anterior to knee anterior to hip (ant. to everything)
• Loading response posterior to ankle posterior to knee anterior to hip
• Midstance anterior to ankle anterior to knee posterior to hip
• Terminal stance anterior to ankle anterior to knee posterior to hip
• Preswing anterior to ankle posterior to knee posterior to hip

• Hamstrings are most active at swing phase

• The hamstring functions to decelerate the thigh in late swing

• During walking, the center of gravity has an up/down motion of about an inch (2.5 cm) – may increase with walking faster.

• At heel strike the subtalar joint EVERTS – this unlocks the midfoot so that it can roll easily during midstance. The tib post tendon then activates to INVERT the subtalar joint during mid-terminal stance and this locks the midfoot so that push off is against a rigid lever.

• The axis of the calcaneocuboid and talonavicular joints are aligned when the subtalar joint is everted – this allows midfoot motion and the foot to accommodate the loading. As the subtalar joint is inverted by tib post, the axis of the calcaneocuboid and talonavicular joints are divergent, and thus the midfoot is locked

• A flexion moment of the hip exists during heelstrike (the joint reaction force passes anterior to the hip); there is a flexion moment only at the beginning of the stance phase – the joint reaction force then moves BEHIND the hip at midstance. There is a flexion moment to the hip during all of the swing phase.

• Center of gravity is highest at midstance; probably lowest during loading response (all joints are flexed)

• At heelstrike, tib ant, EDL (not EHL), hamstrings, and quadriceps are all active. The hip flexors are not.

• Increased energy consumption for 3 point crutch walking is about 50% (1.5 x normal)

• Bone: 70% mineral/inorganic material – vast majority of this is hydroxyapatite

• 25-22% organic matrix (osteoid) – vast majority of this osteoid is collagen and other proteins (98%) The remaining 2% are cells.
• 5-8% water

• Approximately 90% of the organic matrix of bone is type I collagen – the remainder consists of noncollagenous matrix proteins. (some say 95%)

• Bone is anisotropic

• Matrix vesicles in bone contain phosphatases such as alkaline phosphatase and pyrophosphatase, which remove calcification inhibitors and provide the phosphate ions that allow mineral precipitation to take place. Pyrophosphatases therefore enhance mineralization.

• Mineralization occurs in the gaps and pores within collagen fibers

• The function of alk phosphatase is not entirely clear – it may be required to provide PO4 and render the matrix calcifiable. It has some sort of role in mineralization.

• While osteoblasts/cytes have alk phosphatase to aid in mineralization, osteoclasts have acid phosphatase. The osteoclasts interact with bone at the CLEAR ZONE and RUFFLED BORDER. The clear zone is where the osteoclast attaches – it surrounds and seals off the area where the bone is to be resorbed. Intracellular carbonic anhydrase produces acid which is used to degrade bone underneat this area.

• Osteons are 250 microns in diameter;

• Volkmann canals connect the haversion canals which run up the center of the osteon.

• For blood flow in diaphyseal bone – inner 2/3 from intraosseous (centrifugal); outer 1/3 from periosteum (centripetal). All the vessels, including the cortical capillaries drain to intramedullary venous sinusoids inside the bone (centripetal)

• In experimental conditions, the average angle of a spiral fracture is approximately 30 degrees

• In the hypertrophic cartilage zone, the glycogen is consumed till depleted, and the condrocytes synthesize alk phosphatase – so you’d expect increase alk phosphatase, decreased glycogen.

Last Updated on January 25, 2026 by Christian Veillette