1. General Principles & Basic Science MCQs

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.

DISCUSSION: Nonbloody effusions that are greater than 1 mL are readily detected by MRI. They appear black on T1-weighted images and white on T2-weighted images. A sanguinous effusion is seen as white on T1-weighted images and black on T2-weighted images.

DISCUSSION: Patients who have skin and soft-tissue infections caused by CA-MRSA often describe the lesion as a spider bite. The cytotoxin Panton-Valentine leukocidin that is produced by many strains of CA-MRSA causes tissue necrosis, resulting in rapid development of an abscess and the appearance of a spider bite. Patients with psoriasis have thick, red skin with flaky, silver-white patches. Tinea corporis is a general term for a cutaneous fungal infection. The lesion appears as a well-demarcated erythematous plaque with a raised border and central hypopigmentation, giving it a ring-like appearance. Primary infection with herpes simplex virus can produce constitutional symptoms with burning, tingling, or stinging at the site. Grouped vesicles with clear fluid 1 to 2 mm in size form on an erythematous base and then rupture, leaving moist ulcers or crusted plaques.

The patient has undergone a Latarjet procedure as shown in the radiographs. After harvesting the coracoid graft, care must be taken to not place too much tension on or dissect excessively near the musculocutaneous nerve. The nerve is encountered 5 cm distal to the coracoid as it enters the conjoint tendon. The lateral antebrachial cutaneous nerve is the terminal branch of the musculocutaneous nerve and; therefore, injury can cause decreased sensation in the lateral forearm.

DISCUSSION: Nutritional rickets is associated with decreased dietary intake of Vitamin D, resulting in low levels of Vitamin D that result in decreased intestinal absorption of calcium and low to normal serologic levels of calcium. To boost serum calcium levels, there is a compensatory increase in PTH and bone resorption, leading to increased alkaline phosphatase levels. REFERENCES: Brinker MR: Cellular and molecular biology, immunology, and genetics in orthopaedics, in Miller MD (ed): Review of Orthopaedics, ed 3. Philadelphia, PA, WB Saunders, 2001, pp 81-94. Pettifor J: Nutritional and drug-induced rickets and osteomalacia, in Farrus MJ (ed): Primer on the Metabolic Bone Diseases and Disorders of Mineral Metabolism, ed 5. Philadelphia, PA, Lippincott Williams and Wilkins, 2003, pp 399-466. Einhorn TA: Metabolic bone disease, in Einhorn TA, O'Keefe RJ, Buckwalter JA (eds): Orthopaedic Basic Science: Foundations of Clinical Practice, ed 3. Rosemont, IL, American Academy of Orthopaedic Surgeons, 2006, in press.

The most likely diagnosis is chronic multifocal osteomyelitis. This is a culture-negative polyostotic disease that is most commonly found in young people. The treatment of choice is anti-inflammatory drugs. The pathology does not suggest eosinophilic granuloma. Antiviral therapy, broad-spectrum antibiotics, and surgical resection are not indicated for this disease.

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.

Children with symptomatic severe torsional malalignment of the lower extremity and patellofemoral pathology show excessive femoral anteversion and external tibial torsion on physical examination and analysis of gait. The functional effect of this torsional malalignment is centered about the knee joint. If nonsurgical management fails to alleviate patellofemoral pain, definitive surgical treatment should consist of corrective osteotomies, including internal rotation of the distal part of the tibia or external rotation of the femur, or both. Patients with surgical correction by osteotomy show an improved gait pattern and appearance of the extremity and a marked decrease in knee pain.

VDDR I is a deficiency of an enzyme predominantly found in the kidney. VDDR II is caused by an inactivating mutation of the receptor for 1,25 (OH)2 vitamin D3.VDDR I is a deficiency of 1a-hydroxylase [converts 25(OH)D to 1a,25(OH)2D3].Lab tests show hypocalcemia, secondary hyperparathyroidism, elevated alkaline phosphatase (ALP) and low or undetectable calcitriol in the presence of adequate 25(OH)D levels. VDDR II or hereditary vitamin D resistant rickets (HVDRR) (autosomal recessive) is an inactivating mutation in the vitamin D receptor (VDR). Lab tests show low serum calcium and phosphate, elevated ALP and secondary hyperparathyroidism. Serum 25(OH)D values are normal and the 1,25(OH)2D levels are elevated (key difference from VDDR I).Malloy et al. reviewed genetic disorders in vitamin D action. They state that VDDR I is an inborn error of vitamin D metabolism coded by the gene CYP27B1. Children with VDDR I present with joint pain/deformity, hypotonia, muscle weakness, growth failure, and hypocalcemic seizures or fractures in early infancy. Treatment is with calcitriol or 1a-hydroxyvitamin D (NOT cholecalciferol). Children with VDDR II present with bone pain, muscle weakness, hypotonia, hypocalcemic convulsions, growth retardation, severe dental caries or teeth hypoplasia. Affected children are resistant to therapy and supra-physiologic doses of all forms of vitamin D.Illustration A shows the differences between VDDR I and VDDR II. Incorrect Answersin the kidney). The liver enzyme vitamin D 25-hydroxylase (found in hepatocytes) is not responsible for VDDR. VDDR II is caused by an inactivating mutation (rather than an activating mutation).

The median survival of patients after discovery of bone metastasis from primary lung carcinoma is shorter compared with other primary sites.

EXPLANATION: The question refers to a giant cell tumor of bone, which is characterized by a uniform distribution of mononuclear stromal cells and multinucleated giant cells.

Brown-Sequard syndrome is an incomplete spinal cord injury that involves damage unilaterally to the cord, most commonly from penetrating trauma. The motors fibers of the cord decussate within the brainstem so the motor deficit is ipsilateral to the injury; whereas, the pain and temperature fibers cross midline immediately on entering the cord so that the sensory deficit is contralateral to the injury. This patient was shot in the left side, thus he would have weakness of the left upper and lower extremity with diminished pain and temperature sensation on the right side of the body. Response 3 describes opposite symptoms that would result from a right-sided injury. Response 1 describes a central syndrome with greater upper than lower extremity involvement. Response 2 is an anterior cord syndrome with only preservation of the posterior columns of the cord. Response 4 describes a C6 root injury.

DISCUSSION: Water is the most abundant component of articular cartilage with a wet weight of 65% to 80%. Collagen accounts for 10% to 20% of the wet weight. Proteoglycans comprise 10% to 15% of the wet weight. The remainder of the wet weight is made up of other collagens, noncollagenous proteins, and chondrocytes.

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.

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.

In patients who have radial nerve dysfunction associated with a closed humeral fracture, nerve function usually will return to normal without surgical exploration. If clinical findings or electromyographic studies show no improvement at 3 months, surgical exploration and repair can be performed. Tendon transfers are performed if nerve repair is deemed unsuccessful.

The arrow points to the biceps femoris, which is inserted onto the fibula. The biceps femoris lies at the posterolateral aspect of the thigh. The semimembranosus and the semitendinous lie at the posterior medial aspect of the thigh.

DISCUSSION: Growth factors are proteins secreted by cells that can act on target cells to produce certain biologic actions. These actions can be described as autocrine, paracrine, and endocrine. Autocrine actions are those in which the growth factor influences an adjacent cell of its origin or identical phenotype. Paracrine actions are those in which the protein influences an adjacent cell that is different in its origin or phenotype. Endocrine actions are those in which the factor influences a cell located at a distant anatomic site.

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.

DISCUSSION: Risk factors for sternoclavicular septic arthritis include IV drug abuse, diabetes, and HIV. According to the reference by Ross et al, Staphylococcus aureus accounts for 49% of infections. SC joint arthritis accounts for 1% of septic arthritis in the general population but 17% in the IV drug abuse population. Symptoms include spontaneous swelling with the appearance of joint subluxation and localized pain. These patients are not uncommonly afebrile.