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 221
Topic: Biomechanics & Biomaterials
A 45-year-old man has severe pain in both feet after his boots become wet while hunting. Examination 3 hours after the onset of symptoms reveals that his feet are cold to touch and the skin appears blanched. Management should consist of
Correct Answer & Explanation
. rapid rewarming in a footbath at 104.0 degrees F to 107.6 degrees F (40 degrees C to 42 degrees C).
Explanation
The patient has frostbite involving both feet. Rapid rewarming in a protected environment is the initial treatment. A footbath with water at 104.0 degrees F to 107.6 degrees F (40 degrees C to 42 degrees C) is ideal.
Question 222
Topic: Biomechanics & Biomaterials
What is the dominant component of articular cartilage extracellular matrix by weight?
Correct Answer & Explanation
. Water
Explanation
DISCUSSION: Articular cartilage is a highly organized viscoelastic material, and load transmission depends on the specific composition of the extracellular matrix. Articular cartilage is devoid of neural, lymphatic, and blood vessel tissue. The extracellular matrix consists of water, proteoglycans, and collagen. Water comprises most of the wet weight (65% to 80%). Type II collagen comprises 95% of the collagen. The collagen and proteoglycan (keratan sulfate and chondroitin sulfate) matrix and its high water content are responsible for the mechanical properties of the articular cartilage. REFERENCES: Buckwalter JA, Mankin HJ: Articular cartilage: Degeneration and osteoarthritis, repair, regeneration, and transplantation. Inst Course Lect 1998;47:487-504. Koval KJ (ed): Orthopaedic Knowledge Update 7. Rosemont, IL, American Academy of Orthopaedic Surgeons, 2002, pp 3-18.
Question 223
Topic: Biomechanics & Biomaterials
Gamma ray irradiation for sterilization of ultra-high molecular weight polyethylene in an oxygen environment can have what effect on the material?
Correct Answer & Explanation
. Decrease mechanical strength
Explanation
Gamma irradiation of Ultrahigh Molecular Weight Polyethylene leads to free oxidation with resultant breaking of polymer chains, changes in the crystalline structure, and deterioration of the mechanical properties of the polymer. Fracture toughness, fatigue strength, and mechanical strength all decrease while the wear rate of irradiated UHMWPE increases. Stiffness is another matter. Initially after irradiation, crosslinking and stiffness actually increase, but in the long term stiffness will decrease as crosslinks continue to break secondary to irradiation induced oxidative damage.
Question 224
Topic: Biomechanics & Biomaterials
Which of the following combinations of total hip arthroplasty components places the patient at the highest risk for mechanically assisted crevice corrosion (trunnionosis) at the head-neck junction?
Correct Answer & Explanation
. A 28-mm ceramic head on a titanium alloy stem
Explanation
Trunnionosis (taper corrosion) is exacerbated by mixed metal junctions (e.g., CoCr head on Ti stem), large head diameters, and small/thin trunnions. A large CoCr head increases the lever arm and toggle at the head-neck junction, while a small titanium trunnion offers less surface area for interference fit, significantly increasing the risk of fretting and crevice corrosion.
Question 225
Topic: Biomechanics & Biomaterials
Figure 40 shows the plain radiograph of a 30-year-old woman who has had a long history of standing bilateral anterior knee pain and a sense of patellar instability without frank dislocation. Nonsurgical management consisting of anti-inflammatory drugs and physical therapy has failed to provide relief. Examination reveals full range of motion of both knees, with moderate patellofemoral crepitance. Patellar apprehension and patellar grind tests are positive. The Q-angle measures 20°. Management should now consist of
Correct Answer & Explanation
. bilateral lateral releases and anteromedialization of the tibial tubercles.
Explanation
DISCUSSION: The history, physical examination, and radiographs indicate that the patellofemoral pain is most likely caused by excessive lateral patellar pressure and patellar maltracking. Because the radiographs reveal the lateral tilt of the patella and lateral subluxation, the treatment of choice is bilateral lateral releases with anteromedialization of the tibial tubercles. This procedure corrects not only the excessive lateral patellar pressure, but also the lateral subluxation. The use of patella-stabilizing braces or taping may provide temporary relief, but these implements are not well-tolerated and they will not change the underlying biomechanics of the knee. Simple lateral release is indicated for isolated lateral tilt, but it does not correct the lateral subluxation. The use of thermal capsular shrinkage for the medial retinaculum has not been proven to provide long-term correction of the deformity.
Question 226
Topic: Biomechanics & Biomaterials
During total hip arthroplasty, which characteristic of irradiated (10 Mrad) and subsequently melted highly cross-linked polyethylene should provide a more wear-resistant construct than traditional gamma-irradiated (2.5-4 Mrad)-in-air polyethylene mated with the same head?
Correct Answer & Explanation
. Resistance to adhesive wear
Explanation
DISCUSSION: Highly cross-linked polyethylene makes material resistant to adhesive wear. Abrasive wear from third bodies does not decrease wear. The fatigue strength of this material is inferior to traditional polyethylene, and its resistance to creep is the same, if not lower, than that of traditional polyethylene.
Question 227
Topic: Biomechanics & Biomaterials
Figure 19 shows the current radiograph of a 48-year-old man who reports hip pain and marked difficulty walking after undergoing revision of a failed total hip replacement 2 years ago. What is the mechanism of failure?
Correct Answer & Explanation
. Fatigue
Explanation
DISCUSSION: Fatigue from repetitive loading of the stem with the distal aspect well-fixed resulted in stem failure. If the stem had loosened, it would not have broken. Crevice corrosion occurs at a taper interface; galvanic corrosion occurs at the junction of two metals of differing electrochemical potentials, not along a uniform portion of the implant. REFERENCES: Simon SR (ed): Orthopaedic Basic Science. Rosemont, IL, American Academy of Orthopaedic Surgeons, 1994, pp 449-486. Gruen TA, McNiece GM, Amstutz HC: “Modes of Failure” of cemented stem-type femoral components: A radiologic analysis of loosening. Clin Orthop 1979;141:17-27.
Question 228
Topic: Biomechanics & Biomaterials
A 55-year-old female presents with acute onset of medial knee pain and a 'pop' while squatting. MRI reveals a medial meniscus posterior root tear. Biomechanically, what is the consequence to the knee joint in this condition?
Correct Answer & Explanation
. Loss of hoop stresses, resulting in biomechanics equivalent to a total meniscectomy
Explanation
A posterior root tear of the medial meniscus leads to meniscal extrusion and complete loss of circumferential hoop stresses. Biomechanically, this is equivalent to a total meniscectomy, leading to decreased contact area and significantly increased peak contact pressures in the medial compartment, which can lead to rapid cartilage degeneration and subchondral insufficiency fractures.
Question 229
Topic: Biomechanics & Biomaterials
A 45-year-old active male has symptomatic isolated medial compartment osteoarthritis with a varus mechanical axis. A medial opening wedge high tibial osteotomy (HTO) is planned. To maintain native sagittal plane kinematics, where should the osteotomy gap be larger?
Correct Answer & Explanation
. Anterior cortex
Explanation
In a medial opening wedge HTO, the posterior gap should be approximately twice the anterior gap to maintain the native posterior tibial slope. Equal or larger anterior opening inappropriately increases posterior tibial slope, which can alter knee kinematics and load the ACL.
Question 230
Topic: Biomechanics & Biomaterials
When polyethylene is exposed to radiation and subsequently heated, certain chemical changes occur in the material. Which of the following statements best describes these changes?
Correct Answer & Explanation
. The process improves (lowers) the wear rate but may increase the risk of fracture.
Explanation
DISCUSSION: Exposure of polyethylene to radiation and then heating it to quench the free radicals leads to a cross-linked material. It converts a high molecular weight polyethylene macromolecule to an interpenetrating network structure of polymer chains. The ductility of the material is decreased, hence the greater risk of fracture. While the wear rate (measured as fewer and smaller particles) against a smooth counterface is markedly reduced, cross-linked polyethylene has shown a larger increase in wear rate when a rougher counterface is used compared to noncross-linked material. Due to reduced mechanical strength, highly cross-linked polyethylene is less resistant to abrasive wear. REFERENCE: Pellicci PM, Tria AJ Jr, Garvin KL (eds): Orthopaedic Knowledge Update: Hip and Knee Reconstruction 2. Rosemont, IL, American Academy of Orthopaedic Surgeons, 2000, pp 32-33.
Question 231
Topic: Biomechanics & Biomaterials
During total hip arthroplasty, what characteristic of irradiated (10 Mrad) and subsequently melted highly cross-linked polyethylene should provide a more wear-resistant construct than traditional gamma- irradiated (2.5-4 Mrad)-in-air polyethylene mated with the same head?
Correct Answer & Explanation
. Resistance to adhesive wear B. Resistance to abrasive wear C. Resistance to fatigue wear
Explanation
DISCUSSION:Highly cross-linked polyethylene makes material resistant to adhesive wear. Abrasive wear from third bodies does not decrease wear. The fatigue strength of such material is inferior to that of traditional polyethylene, and its resistance to creep is the same, if not lower, than that of traditional polyethylene.
Question 232
Topic: Biomechanics & Biomaterials
The acceleration of an object under the influence of a force depends directly on the mass of the object. The angular acceleration of an object under the influence of a moment depends directly on the
Correct Answer & Explanation
. mass moment of inertia.
Explanation
Discussion: Similar to the action of a force, a moment tends to angularly accelerate an object in a manner proportional to a quantity related to the mass of the object. The concept of a more massive object requiring a larger force to cause the same straight line acceleration is straightforward. The concept for changing angular velocity is similar but not identical. The proportional constant between the moment and the resulting angular acceleration is the mass moment of inertia, which depends not only on the mass of the object, but also its distribution. The unit of mass moment of inertia is obtained by multiplying the mass of the object by the square of the distance between an equivalent location of the center of rotation of the object and an equivalent location of the center of mass. Orthopaedic surgeons can change both the amount of mass carried by a limb and the way that the mass is distributed. For example, in applying a cast to the leg, the physician can affect the mass of the cast by the choice of casting material and by the size of the cast. As the mass moment of inertia of the limb increases (e.g., by applying the cast farther down on the leg or using a heavier casting material), the patient will need to exert larger moments to angularly accelerate the leg during gait.
Question 233
Topic: Biomechanics & Biomaterials
The bending stiffness of a slotted stainless steel intramedullary nail will be increased most by
Correct Answer & Explanation
. Changing to a titanium nail
Explanation
To increase stiffness (bending) of an IM nail the diameter must be increased. The moment of inertia is increased by the 4th power of the diameter. The modulus of elasticity of titanium alloy is half of stainless steel. Cross sectional shape influences fixation. Interlocking screws influence rotation.
Question 234
Topic: Biomechanics & Biomaterials
Which of the following statements is true regarding articular cartilage?
Correct Answer & Explanation
. The hydrophilicity of type II collagen molecules gives cartilage its viscoelastic properties
Explanation
The one role of collagen in articular cartilage is to provide the structural framework to resist swelling under high osmotic tissue pressures created by aggrecan. Type II collagen is the predominant type in articular cartilage.Proteoglycans, the most common of which is aggrecan, are produced by chondrocytes and give articular cartilage its hydrophilic properties. Multiple glycosaminoglycans (GAGs), such as chondroitin and keratin can attach to core proteins to form aggrecans. Link proteins then help aggrecans interact with hyaluronic acid. The negative charge of this complex helps create a strong osmotic gradient, which attracts water and increases tissue pressures. Normal aging involves a decrease in the water content of the extracellular matrix while osteoarthritis is associated with increased water content, which leads to loss of strength and elasticity.Chen et al. evaluated the strain and depth related properties of articular cartilage in bovine models. They found that the zero-strain permeability, zero-strain equilibrium confined compression modulus, and deformation dependence constant differed among the layers of cartilage. They suggest that the complex strain-dependent properties of articular cartilage of different thickness and location have clinical implications for tissue engineering.Illustration A is a diagram depicting the extracellular matrix of articular cartilage. Incorrect Answers:
Question 235
Topic: Biomechanics & Biomaterials
A 50-year-old male requires a total patellectomy due to a highly comminuted, unsalvageable patella fracture. How does a total patellectomy fundamentally alter the biomechanics of the knee joint?
Correct Answer & Explanation
. It increases the moment arm of the extensor mechanism.
Explanation
The patella acts as a fulcrum to increase the moment arm of the extensor mechanism. Total patellectomy dramatically decreases this moment arm, mechanically disadvantaging the extensor mechanism. Consequently, it requires 15% to 30% more quadriceps force to achieve full extension, often leading to an extensor lag and earlier fatigue.
Question 236
Topic: Biomechanics & Biomaterials
A 50-year-old female presents with acute medial knee pain and a popping sensation while deep squatting. MRI reveals a medial meniscus posterior root tear. If left untreated, which biomechanical consequence is most likely to occur within the knee joint?
Correct Answer & Explanation
. Loss of hoop stresses simulating a total meniscectomy
Explanation
A posterior root tear completely disrupts the meniscal ring, leading to a loss of hoop stresses and lateral meniscal extrusion. This alters joint kinematics to be functionally equivalent to a total meniscectomy, drastically increasing peak articular contact pressures.
Question 237
Topic: Biomechanics & Biomaterials
Which of the following bearing materials is most resistant to scratching from third-body debris?
Correct Answer & Explanation
. Alumina
Explanation
DISCUSSION: Alumina is the hardest of all the materials listed. Clinical retrieval demonstrates resistance to scratching from third-body debris. REFERENCE: Cooper JR, Dowson D, Fisher J, Jobbins B: Ceramic bearing surfaces in total articular joints: Resistance to third body damage from bone cement particles. J Med Eng Technol 1991;15:63-67.
Question 238
Topic: Biomechanics & Biomaterials
What is the primary reason for including the ilium in the distal fixation of long instrumentation constructs in adult scoliosis? Review Topic
Correct Answer & Explanation
. Better coronal balance
Explanation
Studies have shown that when compared with fixation to the sacrum alone, the success rate of fusion across the lumbosacral junction increases when both the sacrum and ilium are included in the posterolateral construct. Curve correction, coronal balance, and pelvic balance are all attended to within the thoracolumbar spine and are not directly related to the pelvic fixation. Fretting and corrosion are a byproduct of metal-to-metal connections.
Question 239
Topic: Biomechanics & Biomaterials
When formulating antibiotic-loaded polymethylmethacrylate (PMMA) bone cement for the treatment of an infected tibial nonunion, which of the following physical properties is absolutely essential for the admixed antibiotic?
Correct Answer & Explanation
. High lipid solubility
Explanation
The curing of PMMA is an exothermic reaction that can generate temperatures high enough to denature many drugs. Therefore, any antibiotic mixed into bone cement must be highly thermostable (e.g., vancomycin, tobramycin, gentamicin).
Question 240
Topic: Biomechanics & Biomaterials
Which of the following statements best characterizes polymethylmethacrylate (PMMA) when it is used to secure joint components in bone and to distribute the forces evenly across the bone-implant interface?
Correct Answer & Explanation
. Porosity reduction increases the fatigue strength of PMMA.
Explanation
PMMA has no adhesive properties and can be more accurately described as grout than glue. It does not chemically bond to bone or implants; however, mechanical bonding is accomplished with porous or coated components and with cancellous bone. PMMA is approximately three times stronger in compression than in tension. Peak blood levels of monomer are usually seen approximately 3 minutes after the cement is placed. The monomer is cleared by the lungs. Associated hypotension is more closely related to diminished blood volume than to circulating monomer levels. High porosity decreases the tensile and fatigue properties of cement. Manually mixed cement may have porosity as high as 27%. Porosity may be reduced to less than 1% through vacuum mixing or centrifugation of the cement. When adding antibiotics to cement, the compressive and tensile forces are not appreciably decreased, but the overall fatigue strength may be reduced.
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