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Biomechanics & Biomaterials MCQs

Practice Set 11 of 88

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 201

Topic: Biomechanics & Biomaterials

A new composite material is being evaluated for potential use as a prosthetic ligament. In measuring the tensile strength of this material, why is it important to consider and accurately control the loading rate?

. Ligaments are viscoelastic.
. Ligaments are elastic.
. Ligaments experience creep.
. Ligaments undergo stress relaxation.
. Strain in ligaments is recoverable.

Correct Answer & Explanation

. Ligaments are viscoelastic.

Explanation

Ligaments are viscoelastic, meaning that their deformation under load depends not only on the magnitude of the load but also on the rate of loading. Under higher load rates (faster loading), natural ligaments are stiffer and stronger. Therefore, it is important in such an experiment to apply a loading rate that is physiologically relevant, ie, that represents typical or worst case activities. Creep refers to deformation of a viscoelastic material over time under constant load. Stress relaxation refers to a decrease in stress over time under a constant displacement.

Question 202

Topic: Biomechanics & Biomaterials

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What is the primary mechanism of wear of polyethylene acetabular components?

. Crevice corrosion
. Oscillatory fretting
. Oxidative degradation
. Adhesion and abrasion
. Fatigue and delamination

Correct Answer & Explanation

. Adhesion and abrasion

Explanation

Although previous theories on acetabuIar wear implicated fatigue cracking and delamination which is a major mode of polywear in knees, the primary mechanism of wear of polyethylene acetabular components has been shown to be adhesion and abrasion. In an analysis of 128 componenets retrieved at autopsy or revision surgery, wear appeared to occur mostly at the surface of the components and to be due to large strain plastic deformation and orientation of the surface layers into fibrils that subsequently ruptured during multidirectional motion. It was also shown conclusively that 32 mm displayed significantly more wear (volumetric wear) than with either 22 or 26/28 mm heads ( 1 mm increase in size increased volumetric wear by 10%). The wear at the articulating surface was characterized by highly worn polished areas superiorly and less worn areas inferiorly separated by a ridge. Abrasion was very common, occurring after adhesion and plastic deformation of poly fibrils, and abrasion secondary to third body wear. As well, wear rates decreased with longer survival of components, indicating a "wearing in" phenomenon, arguing against oxidative and fatigue wear. Crevice corrossion = occurs in fatigue cracks with low 02 tension (under screw heads,etc.) Oscillatorry fretting = cyclical outer surface abrading from small movements. Fatigue and delamination = predominant in total knees, where stresses are maximum just below the surface of the poly, causing fatigue over time with susequent delamination. In contrast, hip wear occurs primarily at the surface of the poly.

Question 203

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?

. Increase stiffness
. Increase fracture toughness
. Increase fatigue strength
. Decrease mechanical strength
. Decrease wear rate

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 204

Topic: Biomechanics & Biomaterials

A hip compression screw is placed in a test jig and a bending load is applied to the tip of the screw. After the load is released, the screw returns completely to its original shape. What is this type of deformation called?

. Plastic
. Elastic
. Fatigue
. Creep
. Torsion

Correct Answer & Explanation

. Elastic

Explanation

When an implant is loaded below the yield point, by definition, it undergoes elastic deformation, meaning that all of the deformation recovers when the load is removed. If it is loaded above the yield point, then plastic or permanent deformation occurs. Fatigue is the gradual accumulation and progression of cracks in the material, which, after many cycles of loading, can lead to gross failure. Creep is the gradual accumulation of permanent (plastic) deformation over time, which may occur with polymeric materials but is not typical of metals or ceramics. Torsion refers to a torque being applied about the long axis of a bone or an implant. In general, the materials and dimensions of an implant are chosen to avoid plastic deformation or fatigue failure during typical clinical use.

Question 205

Topic: Biomechanics & Biomaterials

Two years ago, a 63-year-old man underwent right total hip arthroplasty (THA) with a modular femoral head-neck and neck-stem prosthesis (a photograph of the removed implant is shown in Figure 181). He now has increasing hip pain. Radiographs reveal a stable hip arthroplasty and elevated serum cobalt and chromium levels. MR imaging is obtained, and, based on these findings, the patient’s hip is revised. Which corrosion type likely is responsible for this THA failure?

. Galvanic
. Pitting
. Fretting
. Crevice

Correct Answer & Explanation

. Galvanic

Explanation

DISCUSSIONMicromotion at the femoral head-neck or stem-neck junction can lead to fretting corrosion. Fretting corrosion is among the most common causes of failure in modular components. Modularity gives surgeons additional intraoperative flexibility but has resulted in corrosion-related failure and an implant recall. Although titanium and cobalt-chrome contain a protective surface oxide layer, continued micromotion at the modular junction may disrupt the protective layer, resulting in fretting corrosion. This may eventually lead to excessive metal ion formation and painful synovitis that necessitates a revision procedure. Galvanic corrosion is attributable to a mismatch in electrochemical gradients between dissimilar metals. Crevice or pitting corrosion occurs in fatigue cracks because of differences in oxygen tension.

Question 206

Topic: Biomechanics & Biomaterials

What proteinaceous compound binds to hyaluronic acid to function as an effective boundary molecular layer in articular cartilage?

. Lubricin
. Vitronectin
. Aggrecan
. Chondroitin sulfate

Correct Answer & Explanation

. Lubricin

Explanation

Hyaluronic acid (HA) is abundant in cartilage and synovial fluid and is thought to be integral to joint lubrication, although its role is not clearly understood. HA binds to lubricin, a glycoprotein, creating a cross-linked network. Boundary lubrication occurs when the fluid film has been depleted and the contacting bearing surfaces are separated only by a boundary lubricant of molecular thickness, which prevents excessive bearing friction and wear. In articular cartilage, this monolayer of glycoprotein is adsorbed on each of the opposing articular surfaces. Friction experiments in a porcine model have shown that with compression, HA diffuses out of the cartilage and becomes physically trapped and constricted by the collagen network at the interface. This in effect creates a "boundary lubricant." Vitronectin is a glycoprotein similar in the N and C terminal to lubricin. It does not bind to HA. Aggrecan is the second-most-common protein by dry weight of cartilage extracellular matrix. Aggrecan interacts with HA and link proteins to create a proteoglycan aggregate that attracts water to cartilage and gives the tissue its viscoelastic properties. Chondroitin sulfate contributes to matrix proteoglycan structure rather than boundary lubrication.

Question 207

Topic: Biomechanics & Biomaterials
Clinical evidence suggests that grafts for replacing a torn anterior cruciate ligament often stretch after surgery. What is the most probable mechanism for this behavior?
. Gross failure at the attachment sites
. Fatigue failure of the ligament tissue
. Creep of the graft material
. Water absorption by the graft material
. Elastic stretch of collagen fibers

Correct Answer & Explanation

. Creep of the graft material

Explanation

DISCUSSION: The stretching of the graft occurs over time as the graft is loaded. Time-dependent deformation under load is called creep and is common in viscoelastic materials such as ligament tissue. Creep can occur under both static and cyclic load conditions; time-dependent deformation will occur as long as load is applied to the tissue. Similarly, when a graft is initially tensioned to a given deformation at surgery, the load generated in the graft will decrease over time; this behavior is called stress relaxation and also is indicative of a viscoelastic material.

Question 208

Topic: Biomechanics & Biomaterials
In total knee arthroplasty, in vitro testing has shown that cross-linking can diminish the rate of polyethylene wear by 30% to 80%. What other change in material properties is possible when polyethylene is highly cross-linked?
. Increased ductility
. Increased wettability
. Diminished fatigue strength
. Decreased resistance to abrasive wear

Correct Answer & Explanation

. Diminished fatigue strength

Explanation

The most important concern regarding highly cross-linked polyethylene relates to decreased mechanical properties. Cross-linking results in reduced ductility, tensile strength, and fatigue crack propagation resistance. These problems have not been shown to cause implant failure in the most recent clinical trials, but they remain the most important mechanical issues associated with current material processing methods.

Question 209

Topic: Biomechanics & Biomaterials
Figure 59 shows properties of a material being tested for use as an implant. What is represented by the portion of the stress-strain curve from point A to point B?
. Elastic limit
. Nonproportional behavior
. Plastic behavior
. Elastic behavior
. Fracture point

Correct Answer & Explanation

. Elastic behavior

Explanation

DISCUSSION: The figure is a stress-strain diagram representing specific metal subjected to increasing tensile stress. The portion of the curve from A to B is a straight line demonstrating a proportional increase in strain for each increase in tensile stress. If the stress is removed at any point between A and C, the material will return to its original shape, returning back along the original curve without permanent deformation. This is termed elastic behavior.

Question 210

Topic: Biomechanics & Biomaterials
Which of the following is considered the lowest level that a standard thoracolumbosacral orthosis (TLSO) can immobilize?
. L1-2
. L3-4
. L5-S1
. T9-10
. T11-12

Correct Answer & Explanation

. L1-2

Explanation

Without more distal immobilization such as a thigh extension, the lower two lumbar segments generally show the same or even increased mobility with a TLSO.

Question 211

Topic: Biomechanics & Biomaterials
Sterilization of ultra-high molecular weight polyethylene by gamma irradiation in air will degrade its wear performance because of
. oxidation.
. melting.
. cross-linking.
. corrosion.
. creep.

Correct Answer & Explanation

. oxidation.

Explanation

DISCUSSION: Gamma irradiation has long been used as a sterilization method for polyethylene. Exposure to gamma irradiation causes breakage of the chemical bonds in the polyethylene, and oxidation will occur if the material is subsequently exposed to air. The amount of oxidation and decrease in wear performance is also related to the length of time that the gamma-irradiated polyethylene is exposed to oxygen.

Question 212

Topic: Biomechanics & Biomaterials
  • In a fatigue test, the maximum stress under which the material will not fail, regardless of how many loading cycles are applied, is defined as
. Endurance limit
. Failure stress
. Critical stress
. Yield stress
. Elastic limit

Correct Answer & Explanation

. Endurance limit

Explanation

Fatigue failure occurs with repetitive loading cycles at stress below the ultimate tensile strength. Fatigue failure depends on the magnitude of the stress and number of cycles. If the stress is less than a predetermined amount of stress, called the endurance limit, the material may be loaded cyclically an infinite number of times without breaking.

Question 213

Topic: Biomechanics & Biomaterials
A 27-year-old runner training for his first marathon reports lateral knee pain after an unusually long training run. He states that the most significant pain occurs while running downhill. Examination of the patient while he is lying on the unaffected side reveals increased pain when manual pressure is applied to the lateral femoral epicondylar area during knee range of motion of 30° to 45°. What is the most likely diagnosis?
. Popliteal tendinitis
. Iliotibial band friction syndrome
. Excessive lateral pressure syndrome
. Lateral meniscal tear
. Stress fracture

Correct Answer & Explanation

. Iliotibial band friction syndrome

Explanation

DISCUSSION: Iliotibial band friction syndrome is one of the most common causes of lateral knee pain in runners. It is caused by increased friction between the iliotibial band and the lateral femoral condyle because of increased tension on the lateral structures. It may be caused by a prominence of the lateral epicondyle or a malalignment of the lower extremity in the runner, including genu varum, tibia vara, heel varus and forefoot supination, or compensating pronation. These structural characteristics can couple with relative muscle imbalance and lead to an altered running gait, enhancing friction between the lateral femoral condyle and the iliotibial band. Management is usually nonsurgical, including stretching of the iliotibial band and strengthening of the hip abductor muscles, with occasional use of cortisone injections or iontophoresis. REFERENCES: Noble CA: The treatment of iliotibial band friction syndrome. Br J Sports Med 1979;13:51-54. James SL: Running injuries to the knee. J Am Acad Orthop Surg 1995;3:309-318. James SL, Jones DV: Biomechanical aspects of distance running, in Cavanagh PR (ed): Biomechanics of Distance Running. Champaign, IL, Human Kinetic Books, 1990, pp 249-269.

Question 214

Topic: Biomechanics & Biomaterials
  • What is the primary mechanism of wear of polyethylene acetabular components?
. Crevice corrosion
. Oscillatory fretting
. Oxidative degradation
. Adhesion and abrasion
. Fatigue and delamination

Correct Answer & Explanation

. Crevice corrosion

Explanation

Although previous theories on acetabuIar wear implicated fatigue cracking and delamination which is a major mode of polywear in knees, the primary mechanism of wear of polyethylene acetabular components has been shown to be adhesion and abrasion. In an analysis of 128 componenets retrieved at autopsy or revision surgery, wear appeared to occur mostly at the surface of the components and to be due to large strain plastic deformation and orientation of the surface layers into fibrils that subsequently ruptured during multidirectional motion. It was also shown conclusively that 32 mm displayed significantly more wear (volumetric wear) than with either 22 or 26/28 mm heads ( 1 mm increase in size increased volumetric wear by 10%). The wear at the articulating surface was characterized by highly worn polished areas superiorly and less worn areas inferiorly separated by a ridge. Abrasion was very common, occurring after adhesion and plastic deformation of poly fibrils, and abrasion secondary to third body wear. As well, wear rates decreased with longer survival of components, indicating a "wearing in" phenomenon, arguing against oxidative and fatigue wear. Crevice corrossion = occurs in fatigue cracks with low 02 tension (under screw heads,etc.) Oscillatorry fretting = cyclical outer surface abrading from small movements. Fatigue and delamination = predominant in total knees, where stresses are maximum just below the surface of the poly, causing fatigue over time with susequent delamination. In contrast, hip wear occurs primarily at the surface of the poly.

Question 215

Topic: Biomechanics & Biomaterials

6 grams each per 40 mg PMMA

. 10 grams each per 40 mg PMMA
. 20 grams each per 40 mg PMMA

Correct Answer & Explanation

. 10 grams each per 40 mg PMMA

Explanation

DISCUSSIONVancomycin and tobramycin do not produce systemic toxicity in doses as high as 10.5 grams of vancomycin and 12.5 grams of tobramycin per 40 mg of PMMA.RECOMMENDED READINGSHake ME, Young H, Hak DJ, Stahel PF, Hammerberg EM, Mauffrey C. Local antibiotic therapy strategies in orthopaedic trauma: Practical tips and tricks and review of the literature. Injury. 2015 Aug;46(8):1447-56. doi: 10.1016/j.injury.2015.05.008. Epub 2015 May 14. Review. PubMed PMID: 26007616.View Abstract at PubMedSpringer BD, Lee GC, Osmon D, Haidukewych GJ, Hanssen AD, Jacofsky DJ. Systemic safety of high-dose antibiotic-loaded cement spacers after resection of an infected total knee arthroplasty. Clin Orthop Relat Res. 2004 Oct;(427):47-51. PubMed PMID: 15552135.View Abstract at PubMedCLINICAL SITUATION FOR QUESTIONS 84 THROUGH 86Figure 84 is the noncontrast MR image obtained at 2 weeks postinjury for a 52-year-old man who was injured after falling from a ladder. His arm is caught in an abducted, externally rotatedposition. There is no sense of dislocation. Initial radiograph findings are normal. The radiologic review reads “Probable superior labral anterior to posterior (SLAP) tear.”

Question 216

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?

. Resistance to adhesive wear B. Resistance to abrasive wear C. Resistance to fatigue wear
. Resistance to creep

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 217

Topic: Biomechanics & Biomaterials
In the setting of a proximal tibial plateau fracture and its repair, which of the following materials is an isotropic material?
. Proximal tibial cortical bone
. Articular cartilage of the tibial plateau
. Stainless steel tibial plate
. Medial or lateral meniscus
. Calcium phosphate cement

Correct Answer & Explanation

. Stainless steel tibial plate

Explanation

DISCUSSION: An isotropic material is one that has similar mechanical properties regardless of the orientation of the material. Examples of isotropic materials include metals, plastics, and methacrylate. Most biologic tissues are anisotropic, meaning their mechanical properties alter depending on the materials’ orientation to the applied stress. REFERENCE: Einhorn TA, O’Keefe RJ, Buckwalter JA (eds): Orthopaedic Basic Science: Foundations of Clinical Practice, ed 3. Rosemont, IL, American Academy of Orthopaedic Surgeons, 2007, pp 58-59.

Question 218

Topic: Biomechanics & Biomaterials
What percentage of bone weight is collagen?
. 5% to 10%
. 20% to 25%
. 50% to 60%
. 70% to 75%
. 90% to 95%

Correct Answer & Explanation

. 20% to 25%

Explanation

Bone is a composite of both inorganic and organic material. The inorganic component of bone comprises 60% to 70% of the tissue, water accounts for 5% to 8%, and the organic matrix makes up the remainder. Collagen accounts for 90% of the organic component and thus 20% to 25% of bone weight. Collagen accounts for the flexibility of bone. The inorganic component of bone is made primarily of calcium and phosphorous, in the analogue of hydroxyapatite, and other ions including sodium, magnesium, and carbonate.

Question 219

Topic: Biomechanics & Biomaterials
The load versus deformation curve of the functional spinal unit (FSU) is made up of the neutral zone, the elastic zone, and the plastic zone. What is the plastic zone of the curve believed to represent?
. Lining up of collagen fibers within the ligaments
. Stretching of elastin within the disk and ligaments
. Transition between flexion and extension
. Reversible elongation of the soft tissues
. Traumatic range of motion, resulting in damage to the soft tissues

Correct Answer & Explanation

. Traumatic range of motion, resulting in damage to the soft tissues

Explanation

Plastic deformation of viscoelastic tissues represents deformation of the soft tissues to the point of failure. The lining up of collagen fibers would be in the “toe region” of the curve, which, in the case of the FSU, would be mainly in the neutral zone. Elastin is a minor contributor to the composition of the ligaments and would be protected by the stiffer collagen fibers. The transition between flexion and extension occurs in the neutral zone, and reversible elongation occurs in the elastic zone.

Question 220

Topic: Biomechanics & Biomaterials

What mechanism is most likely responsible for the initiation of mechanical failure seen at the midstem modular junction of modular revision hip stems?

. Fretting fatigue
. Etching
. Crevice corrosion
. Abrasive wear

Correct Answer & Explanation

. Fretting fatigue

Explanation

The junction fracture is initiated in a fretting-fatigue mechanism and completed by a bending moment. Fretting fatigue occurs when contacting components experience cyclic loads while small oscillatory motion occurs between them. This increases tensile and shear stress, leading to small flaws that result in crack propagation. In the referenced study no evidence of corrosion was seen because there was no etching, pitting, corrosion products, or chloride formation. Etching is a finding seen in corrosion. Abrasive wear occurs when a rough surface glides against a softer surface. It is generally seen in polyethylene wear from bone or cement in third-body wear. Risk factors for stem breakage include high body mass index and lack of proximal bone support at the modular stem-junction area.

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