1. General Principles & Basic Science MCQs

DISCUSSION: Prior to the late 80’s, techniques for transfemoral amputation sacrificed the hip adductor muscles resulting in unopposed abductor forces. Amputation with an abducted femur leads to an increase in side lurch and higher energy consumption. Gottschalk in ’99 showed that myodesis of the adductor magnus through drill holes in the lateral femur preserved maximum muscle force and provided a mechanical advantage for the adductors of the thigh. This resulted in maintenance of the normal anatomic alignment of the femur and a balance between the abductor and adductor mechanisms of the hip, thus providing patients with improved control and easier prosthesis fit.

DISCUSSION: The radial nerve enters the anterior compartment through the intercompartmental fascia on average 10 cm proximal to the radiocapitellar joint. It has never been found to remain in the posterior compartment within the distal arm.

The C1 lateral mass can safely accommodate screw fixation. Trajectory of 10 degrees medial and 22 degrees cephalad was safely applied in a series of 50 patients. Postoperative CT scans confirmed the safe trajectory. The benefit of lateral mass screws is that they can be safely placed despite the existence of an anomalous vertebral artery that could preclude the safe placement of transarticular screws.

DISCUSSION: The radiographs show Ficat and Arlet stage 2 osteonecrosis. The femoral head remains round, and there are sclerotic changes in the superolateral quadrant. Patients with systemic lupus erythematosus are at risk for osteonecrosis because of prednisone use and the underlying metabolic changes associated with the condition (hypofibrinolysis and thrombophilia). MRI is the best diagnostic method for detecting osteonecrosis, with a greater than 98% sensitivity and specificity. For this patient, an MRI can assess the contralateral hip for any involvement and can quantify the extent of the lesion. REFERENCES: Mont MA, Jones LC, Sotereanos DG, Amstutz HC, Hungerford DS: Understanding and treating osteonecrosis of the femoral head. Instr Course Lect 2000;49:169-185. Koval KJ (ed): Orthopaedic Knowledge Update 7. Rosemont, IL, American Academy of Orthopaedic Surgeons, 2002, pp 417-451.

The clinical picture is one of infection and deep venous thrombosis. The MRI scan is consistent with osteomyelitis. Deep venous thrombosis in association with musculoskeletal infection is more common in osteomyelitis caused by methicillin-resistantStaphylococcus aureus. Presenting C-reactive protein levels generally are higher than 6 mg/L and are higher than with other causative organisms. The presence of the Panton-Valentine leukocidin gene encoded in strains of bacteria may explain the deep venous thrombosis.

DISCUSSION: The possibility of an open joint injury should be considered in any patient who has a small periarticular laceration. Failure to promptly diagnose and treat such injuries may lead to septic arthritis. The diagnosis of an open joint is easily made when there is visible communication of the joint through the traumatic wound, or when intra-articular air is present on a radiograph. In the absence of these findings, the diagnosis of an open joint may be established by the saline load test, in which a volume of saline is injected into the joint under sterile conditions. If fluid extravasates through the traumatic wound, the diagnosis of an open joint is established. REFERENCES: Voit GA, Irvine G, Beals RK: Saline load test for penetration of periarticular lacerations. J Bone Joint Surg Br 1996;78:732-733.

To treat depressed tibial plateau fractures, surgeons must elevate the depressed segments, which results in a metaphyseal bone void. This void can be filled with multiple materials, including autograft and allograft bone. The addition of BMP-2 or BMP-7 is not recommended to fill contained bone defects. These materials are indicated for application between bone and soft tissue and they do not provide the volume or strength to fill closed defects. Calcium phosphates have been used to fill these voids, demonstrating improved strength and resistance to subsidence when compared to autograft. Although polymethylmethacrylate is commonly used to fill bone voids in patients with cancer, it is not preferred in the setting of acute fracture because of the exothermic reaction that may compromise local bone healing. Although hydroxyapatite may be degraded by macrophage and giant-cell-mediated processes, the calcium phosphate degrades mainly by an osteoclast-mediated method. The calcium phosphate is not soluble in interstitial fluid, so simple dissolution does not occur.

The safe zone for anterior halo pin insertion is marked laterally by the anterior border of the temporalis muscle (to avoid penetration of this muscle and relative thin cortex of the skull). Medially, the pin should be placed 4.5 cm lateral to the midline to avoid injury to the supraorbital nerve or the frontal sinus. The safe area is marked superiorly by the head equator to avoid cephalad migration of the pin and inferiorly by the supraorbital ridge to prevent displacement or penetration into the orbit.

Allograft bone is one of the most common bone graft substitutes, and is frequently used as a bone graft extender. The sterilization process kills all cells but their three-dimensional structure is retained, offering an osteoconductive scaffold. Tricalcium phosphate, calcium phosphate, and calcium sulfate are also osteoconductive materials with a three-dimensional scaffold similar to native cancellous bone. Bone morphogenetic protein is an inductive protein, which stimulates osteoprogenitor cells to differentiate along a bone-forming lineage.

Linezolid is the first agent of the oxazolidinone group of antibiotics and is very active against methicillin-sensitive Staphylococcus aureus, S. epidermidis, and vancomycin-resistant enterococci. The drug has no gram-negative activity. Linezolid inhibits protein synthesis by blocking formation of the 70S ribosomal translation complex. This mechanism of action is unique to the oxazolidinones.

Dkk-1 and sclerostin are proteins that inhibit the binding of the Wnt molecule to receptors LRP5/6. In the absence of sclerostin and Dkk-1, Wnt binds to its receptor, which in turn inhibits phosphorylation of the β-catenin. The unphosphorylated β-catenin then builds up in the cytoplasm of the cell, allowing it to be transported to the nucleus of the cell. Once in the nucleus, β-catenin will lead to upregulation of a series of proteins involved in osteoblast formation differentiation. Knocking out or inhibiting sclerostin or Dkk-1 results in increased bone mass secondary to constitutive activation of the Wnt/β-catenin pathway. The other responses are not directly affected by Dkk-1 or sclerostin. RANKL and RANK are expressed on osteoblasts and osteoclasts, respectively, and are involved in osteoblast-mediated osteoclast activation. BMPs work through SMADs to cause osteoblastic differentiation, and there is reported crosstalk between the Wnt and BMP pathways (but this is an indirect link). Finally, PTH at physiologic levels binds to osteoblasts, causing a series of events that lead to osteoblast-mediated osteoclast activation and subsequent increased bone resorption.

In the Smith-Petersen approach to the hip, dissection is carried out between the tensor fascia lata laterally (supplied by the superior gluteal nerve) and the sartorius and rectus femoris medially (both supplied by the femoral nerve).

The patient has chronic tibial osteomyelitis that is due to low virulent bacteria. The history and studies do not suggest the need for an amputation or a free-flap procedure. This is a localized tibial infection that is in a healed bone; there is no need to resect the entire area of the tibia bone around the infection. The most appropriate treatment is curettage, debridement of nonviable bone, and placement of absorbable antibiotic beads, followed by a course of IV antibiotics from 1 to 4 weeks and a 6-week course of oral antibiotics. Studies have shown that in cases of localized osteomyelitis that are of low virulence, as little as 1 week of IV antibiotics followed by 6 weeks of oral antibiotics is successful.

Specificity refers to the proportion of individuals who are truly free of the designated disorder who are so identified by the test. Sensitivity refers to the proportion of individuals who truly have the disorder who are so identified by the test. Positive predictive value refers to the proportion of individuals with a positive test who have the disorder. Negative predictive value refers to the proportion of individuals with a negative test who are free of the disorder. Accuracy is the overall ability to identify patients with the disorder (true positives) and without the disorder (true negatives) in the study population.

DISCUSSION: In the cited study, researchers measured tissue fluid pressure within the sciatic nerve in cadaveric specimens using a pressure transducer. The hip and knee were taken through a combination of ranges and found that the clinically relevant increase in pressure happened with the hip flexed at 90 degrees and the knee fully extended. They concluded that increased intraneural pressure was related to excursion of the nerve as linear distance between the greater sciatic notch and the distal leg increase. Hence, according to the question stem, to avoid traction injury, the reverse position should be implemented (hip extension and knee flexion).

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.

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.

Alendronate is a bisphosphonate that inhibits the ruffled border of the osteoclast. When used long term, this class of medication prevents the normal bone remodeling process. Long-term use has recently been shown to be associated with insufficiencyfractures of the femur. Osteonecrosis of the jaw has been described but not in other anatomic locations. Scurvy occurs because of a lack of vitamin C and use of bisphosphonates is not associated with uterine cancer or a detached retina.

DISCUSSION: Operative treatment is indicated for most displaced acetabular fractures to allow early ambulatory function and to decrease the chance of post-traumatic arthritis. Among the various surgical approaches, the Kocher-Langenbach allows direct exposure of both the posterior column and posterior wall. Indications for using this exposure include posterior wall fractures, posterior column fractures, combined posterior wall/posterior column fractures, and simple transverse fractures.

DISCUSSIONNumerous studies have demonstrated efficacy of both IV and topical administration of TXA for decreasing blood loss and transfusion rates. Several studies have shown no significant difference between TXA IV and topical administration in decreasing blood loss or lowering transfusion rates. Inconclusive evidence shows that IV administration of TXA places individuals at higher risk for a thromboembolic event. Both IV and topical TXA are equally effective in decreasing blood loss and minimizing transfusion rates.