This practice set contains high-yield board review questions covering key concepts in Biomechanics & Biomaterials. Each clinical scenario is designed to test your diagnostic and management skills relevant to this subspecialty.
Question 821
Topic: Biomechanics & Biomaterials
Which of the following describes the 'creep' phenomenon in viscoelastic materials like bone or ligaments?
Correct Answer & Explanation
. Immediate deformation under a sudden load.
Explanation
Creep is the time-dependent increase in deformation (strain) of a material when subjected to a constant load (stress) over time. Stress relaxation, conversely, is the time-dependent decrease in stress when a material is held at a constant deformation. Viscoelastic materials exhibit both creep and stress relaxation. Immediate deformation is elastic or plastic response. Fatigue relates to repeated loading. Yield point marks the onset of plastic deformation.
Question 822
Topic: Biomechanics & Biomaterials
What is the primary role of polymethylmethacrylate (PMMA) bone cement in joint arthroplasty?
Correct Answer & Explanation
. To provide direct biological fixation to bone.
Explanation
PMMA bone cement functions as a grout, filling the irregularities between the implant and the bone, and then polymerizes to create a mechanical interlock. It is not a biological adhesive and does not promote osteointegration. It transfers stress between the implant and bone but is not typically considered a primary load-bearing structural component in the same way the implant itself is. While antibiotics can be added to cement, its primary role is mechanical fixation.
Question 823
Topic: Biomechanics & Biomaterials
What is the primary advantage of uncemented (press-fit) components in total joint arthroplasty?
Correct Answer & Explanation
. Lower risk of infection
Explanation
The primary advantage of uncemented components is the potential for long-term biological fixation through osteointegration, where the patient's own bone grows directly onto or into the implant's porous surface. This can provide durable fixation. While it eliminates PMMA heat, it does not provide immediate biological fixation (mechanical press-fit is immediate), and revision surgery can be more complex due to bone ingrowth. Infection risk is not inherently lower.
Question 824
Topic: Biomechanics & Biomaterials
In optimizing the mechanical fixation of a non-locking plate for a diaphyseal osteoporotic humerus fracture, which of the following geometrical modifications to the cortical screw will most significantly increase its pullout strength?
Correct Answer & Explanation
. Increasing the core (minor) diameter while maintaining the outer diameter
Explanation
Pullout strength is the force required to pull a screw out of the bone and is proportional to the shear area of the bone engaged by the threads. The formula for pullout strength shows that it is most highly correlated with the major (outer) diameter of the screw. Increasing the core diameter increases the bending strength of the screw itself but does not increase pullout strength. Decreasing pitch increases the number of threads engaged, which helps, but is less impactful than increasing the major diameter.
Question 825
Topic: Biomechanics & Biomaterials
During open reduction and internal fixation of a periprosthetic femur fracture, an inexperienced surgeon opts to use a standard stainless steel cortical screw to secure a commercially pure titanium locking plate. Which of the following best explains why this combination of dissimilar metals increases the risk of premature implant failure?
Correct Answer & Explanation
. Titanium has a lower modulus of elasticity, causing profound stress shielding of the adjacent stainless steel screw
Explanation
Mixing dissimilar metals (e.g., titanium and stainless steel) in the electrolytic environment of human tissue fluid creates a galvanic cell. Because the metals have different electrochemical potentials, electrons flow from the less noble metal (anode) to the more noble metal (cathode), resulting in accelerated galvanic corrosion of the anodic metal. This compromises structural integrity and releases potentially toxic metallic ions into the surrounding tissues.
Question 826
Topic: Biomechanics & Biomaterials
A surgeon is deciding between a solid and a hollow intramedullary nail of the exact same outer diameter and material for a diaphyseal tibia fracture. Compared to the solid nail, the hollow nail possesses which of the following mechanical characteristics?
Correct Answer & Explanation
. Higher bending rigidity
Explanation
Torsional rigidity is proportional to the polar moment of inertia. For a cylinder, a hollow core reduces the polar moment of inertia compared to a solid cylinder of the identical outer diameter, thus lowering its torsional rigidity. Bending rigidity (determined by the area moment of inertia) is also lower for a hollow nail compared to a solid nail of the exact same outer diameter.
Question 827
Topic: Biomechanics & Biomaterials
A patient presents with localized pain and swelling two years after internal fixation of a distal radius fracture with a stainless steel plate and screws. Radiographs show extensive peri-implant radiolucency. Revision surgery reveals dark grey tissue staining and loosening of one screw, which was found to be inadvertently made of titanium. What type of corrosion primarily caused this phenomenon?
Correct Answer & Explanation
. Pitting corrosion
Explanation
Galvanic corrosion occurs when two electrochemically dissimilar metals (e.g., stainless steel and titanium) are placed in direct physical contact within an electrolytic medium (such as human body fluids). An electrochemical cell is created, and the less noble (more anodic) metal corrodes preferentially and rapidly. This is why mixing different implant metals in a single construct is contraindicated.
Question 828
Topic: Biomechanics & Biomaterials
A 65-year-old patient receives a total hip arthroplasty with a highly cross-linked polyethylene liner. What is the primary purpose of remelting the polyethylene after gamma irradiation during the manufacturing process?
Correct Answer & Explanation
. To increase the degree of cross-linking
Explanation
Gamma irradiation induces cross-linking in ultra-high-molecular-weight polyethylene (UHMWPE) to improve wear characteristics, but it also generates free radicals. If left untreated, these free radicals react with oxygen in vivo, leading to oxidative degradation and subsequent implant failure. Remelting allows trapped free radicals to recombine and be eliminated, though it slightly decreases the mechanical properties (such as yield strength and fatigue resistance) compared to annealing.
Question 829
Topic: Biomechanics & Biomaterials
During tensile testing of a healthy flexor tendon, the stress-strain curve initially exhibits a non-linear 'toe region'. Which microscopic structural characteristic is primarily responsible for this specific region of the curve?
Correct Answer & Explanation
. Rupture of intermolecular cross-links
Explanation
The stress-strain curve of tendons and ligaments typically begins with a non-linear 'toe region' at low strains (up to approximately 2%). This is anatomically due to the straightening, or 'uncrimping', of the naturally crimped type I collagen fibrils. Once straightened, the tendon enters the linear (elastic) region, where stiffness increases significantly until microscopic failure (yield point) and eventual macroscopic rupture occur.
Question 830
Topic: Biomechanics & Biomaterials
A surgeon uses a stainless steel plate with titanium alloy screws for the internal fixation of a diaphyseal fracture. Which of the following types of corrosion is most likely to occur at the screw-plate interface?
Correct Answer & Explanation
. Fretting corrosion
Explanation
Galvanic corrosion occurs when two dissimilar metals with different electrochemical potentials are placed in physical contact within an electrolytic medium (such as blood or interstitial fluid). In this scenario, mixing stainless steel and titanium creates a galvanic cell, resulting in accelerated corrosion of the more anodic metal (typically the stainless steel). To avoid this, orthopedic implants of different metal alloys should generally not be mixed.
Question 831
Topic: Biomechanics & Biomaterials
A modular revision total hip arthroplasty fails due to significant wear and metal debris at the head-neck taper junction. Which type of corrosion is primarily responsible for this specific mode of failure at the modular interface?
Correct Answer & Explanation
. Galvanic corrosion
Explanation
Fretting corrosion occurs at the contact sites between materials under load, such as the modular head-neck taper junction (trunnionosis) in a total hip arthroplasty. Micromotion disrupts the passive protective oxide layer, leading to mechanical wear and subsequent corrosive attack.
Question 832
Topic: Biomechanics & Biomaterials
During mechanical tensile testing of a healthy human anterior cruciate ligament, the initial non-linear 'toe region' observed on the stress-strain curve is primarily due to which of the following phenomena?
Correct Answer & Explanation
. Plastic deformation of collagen cross-links
Explanation
The initial 'toe region' of the stress-strain curve for ligaments and tendons represents the uncrimping or straightening of the naturally wavy collagen fibers. Once the fibers are straight, the curve enters the linear (elastic) region where further stress causes proportional strain.
Question 833
Topic: Biomechanics & Biomaterials
A 65-year-old man requires revision of a total hip arthroplasty that was originally performed using a cobalt-chromium modular head and a titanium alloy femoral stem. Intraoperatively, extensive black, soot-like debris is noted at the modular head-neck taper junction, accompanied by localized soft tissue necrosis. Which of the following is the primary pathophysiologic mechanism responsible for the debris generation and implant degradation at this specific interface?
Correct Answer & Explanation
. Galvanic corrosion directly caused by the continuous electron transfer between two dissimilar metals in a conductive fluid.
Explanation
The scenario describes 'trunnionosis' (corrosion at the modular head-neck junction), which typically presents with black debris and adverse local tissue reactions (ALTR). The primary mechanism driving this is mechanically assisted crevice corrosion (MACC), also known as fretting-crevice corrosion. Micromotion at the taper junction cyclically disrupts the protective passivation (oxide) layer of the metals (fretting). When this occurs in a confined, oxygen-depleted space like a modular taper (a crevice), the local environment becomes acidic and chloride-rich, preventing the repassivation of the metal and leading to rapid, localized continuous corrosion. Although Cobalt-Chromium and Titanium are dissimilar metals, true macroscopic galvanic corrosion is not the primary driver in modern modular junctions because both rely on stable, similar oxide layers with close resting potentials; rather, it is the mechanical disruption (fretting) combined with the confined chemical environment (crevice) that causes catastrophic failure.
Question 834
Topic: Biomechanics & Biomaterials
A 55-year-old man presents with an acute, painful, swollen left knee. Joint aspiration yields synovial fluid with a white blood cell count of 45,000 cells/mm³, consisting of 80% polymorphonuclear leukocytes. Under polarized light microscopy, rhomboid-shaped crystals are visualized within the neutrophils. Which of the following is the most likely composition of these crystals?
Correct Answer & Explanation
. Monosodium urate
Explanation
The presence of rhomboid-shaped crystals in the synovial fluid that are typically weakly positively birefringent under polarized light is characteristic of calcium pyrophosphate dihydrate (CPPD) crystal deposition disease (pseudogout). Monosodium urate crystals, seen in gout, are needle-shaped and strongly negatively birefringent.
Question 835
Topic: Biomechanics & Biomaterials
In a biomechanical study, an intact anterior cruciate ligament (ACL) specimen is subjected to a constant tensile load over 24 hours. The investigators observe a progressive increase in ligament deformation (length) over time despite the load remaining unchanged. This viscoelastic phenomenon is known as:
Correct Answer & Explanation
. Stress relaxation
Explanation
Creep is the continuous deformation of a viscoelastic material when subjected to a constant, sustained load. Stress relaxation (load relaxation) is the decline in stress (force) observed when a material is stretched to and held at a constant length. Hysteresis represents the energy lost (as heat) between the loading and unloading curves of a viscoelastic material.
Question 836
Topic: Biomechanics & Biomaterials
A 65-year-old woman is scheduled to undergo a total hip arthroplasty. The surgeon plans to use a highly cross-linked polyethylene (HXLPE) liner. Which of the following describes the primary tribological and mechanical trade-off associated with highly cross-linked polyethylene compared to conventional ultra-high-molecular-weight polyethylene (UHMWPE)?
Correct Answer & Explanation
. Decreased volumetric wear but decreased fracture toughness
Explanation
Highly cross-linked polyethylene (HXLPE) was developed to reduce volumetric wear and the subsequent risk of particle-induced osteolysis, which is its primary clinical advantage. However, the cross-linking process (typically via gamma or electron beam irradiation) followed by thermal treatment (melting or annealing) alters the material properties. The primary trade-off is a reduction in mechanical properties, specifically decreased fracture toughness, reduced fatigue resistance, and decreased ultimate tensile strength compared to conventional UHMWPE.
Question 837
Topic: Biomechanics & Biomaterials
A 65-year-old man who underwent a primary total hip arthroplasty using a titanium stem and a cobalt-chromium modular head 5 years ago presents with new-onset groin pain. Laboratory tests show elevated serum cobalt levels, while chromium levels are mildly elevated. MRI reveals a solid-cystic pseudotumor around the hip joint. The primary mechanism responsible for the release of these metal ions at the modular head-neck junction is best described as:
Correct Answer & Explanation
. Galvanic corrosion
Explanation
The release of metal ions at the modular head-neck junction (trunnion) of a total hip arthroplasty is primarily due to mechanically assisted crevice corrosion (MACC), also known as fretting corrosion. Micromotion at the modular junction continuously disrupts the protective passivation layer of the metals, leading to subsequent corrosive attack and the release of metal debris and ions (particularly cobalt). While galvanic corrosion can technically occur when two dissimilar metals are in contact, mechanically assisted crevice corrosion is the overwhelmingly dominant mode of failure driving trunnionosis in modern modular implants.
Question 838
Topic: Biomechanics & Biomaterials
A new orthopedic polymer implant is being evaluated for use in load-bearing applications. When subjected to a constant load over a prolonged period of several months, the material demonstrates a progressive increase in strain and deformation. Which of the following viscoelastic properties best describes this phenomenon?
Correct Answer & Explanation
. Stress relaxation
Explanation
Creep is the progressive deformation of a material when it is subjected to a constant force or stress over time. Stress relaxation is the decrease in stress over time when a material is held at a constant deformation. Hysteresis refers to the energy lost (usually as heat) during the loading and unloading cycle of a viscoelastic material. Fatigue failure occurs from repetitive cyclical loading below the ultimate tensile strength. Anisotropy means a material's mechanical properties are dependent on the direction of loading.
Question 839
Topic: Biomechanics & Biomaterials
To reduce wear rates in total hip arthroplasty, ultra-high-molecular-weight polyethylene (UHMWPE) components are often subjected to highly cross-linking processes using irradiation. Which of the following best describes the resulting biomechanical changes in the highly cross-linked polyethylene?
Correct Answer & Explanation
. Decreased volumetric wear and increased fracture toughness
Explanation
Highly cross-linked polyethylene (HXLPE) undergoes irradiation to form cross-links between polymer chains, which significantly decreases abrasive and volumetric wear. However, this process alters the mechanical properties of the material, leading to decreased fracture toughness, reduced ultimate tensile strength, and reduced fatigue resistance. A post-irradiation melting or annealing process is typically used to eliminate free radicals and prevent oxidation.
Question 840
Topic: Biomechanics & Biomaterials
Viscoelastic materials such as ligaments and tendons exhibit time-dependent mechanical behavior. During a laboratory experiment, a human cadaveric patellar tendon is subjected to a constant, sustained tensile load, resulting in a progressive increase in length over time. This specific biomechanical phenomenon is best described as:
Correct Answer & Explanation
. Stress relaxation
Explanation
Creep is defined as the progressive deformation (increase in strain) of a viscoelastic material when it is subjected to a constant load (stress) over a period of time. In contrast, stress relaxation occurs when a material is held at a constant length (constant strain), and the force required to maintain that length decreases over time. Hysteresis represents the energy lost as heat during the loading and unloading cycle of a viscoelastic material.
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