FNB MOCK TEST 2 Answers & Explanations. Q. 1 Ratio of body weight lever arm to that of abductor musculature is A. 1:2 B. 2:1 C. 2.5: 1 D. 1:2.5 Explanation: The ratio of the length of the lever arm of the body weight to that of the abductor musculature is about 2.5 : 1. The force of the abductor muscles must approximate 2.5 times the body weight to maintain the pelvis level when standing on one leg. Q.2 DORR classification is used to A. Classify femoral stem B. Classify proximal femur C. Classify cemented stem D. Classify Uncemented stem Explanation: Dorr et al. proposed a radiographic categorization of proximal femurs based on their shape and correlated those shapes with measurements of cortical thickness and canal dimensions. Type A femurs have thick cortices on the anteroposterior view and a large posterior cortex seen on the lateral view. The narrow distal canal gives the proximal femur a pronounced funnel shape or “champagne flute” appearance. The type A femur is more commonly found in men and younger patients and permits good fixation of either cemented or cementless stems. Type B femurs exhibit bone loss from the medial and posterior cortices, resulting in increased width of the intramedullary canal. The shape of the femur is not compromised, and implant fixation is not a problem. Type C femurs have lost much of the medial and posterior cortex. The intramedullary canal diameter is very wide, particularly on the lateral radiograph. The “stovepipe”-shaped type C bone is typically found in older postmenopausal women and creates a less favorable environment for implant fixation. Q. 3 Highest stresses in the acetabular trabecular bone is seen with A. When a thin-walled, polyethylene acetabular component is used and when the subchondral bone has been removed B. When a thick-walled, polyethylene acetabular component is used and when the subchondral bone has been removed C. When a metal backed polyethylene acetabular component is used and when the subchondral bone has been removed D. When a thin-walled, polyethylene acetabular component is used and when the subchondral bone has not been removed Explanation: On the pelvic side, finite analysis has indicated that with the use of a cemented polyethylene cup, peak stresses develop in the pelvic bone. A metal-backed cup with a polyethylene liner reduces the high areas of stress and distributes the stresses more evenly. Similar studies have indicated that increased peak stresses develop in the trabecular bone when the subchondral bone is removed and that decreased peak stresses develop when a metal-backed component is used. The highest stresses in the cement and trabecular bone develop when a thin-walled, polyethylene acetabular component is used and when the subchondral bone has been removed. A thick-walled polyethylene cup of 5 mm or more, as opposed to a thin-walled polyethylene cup, tends to reduce the stresses in the trabecular bone, similar to the effect of the metal-backed cup. The preservation of subchondral bone in the acetabulum and the use of a metal-backed cup or thick walled polyethylene cup decrease the peak stress levels in the trabecular bone of the pelvis. Q. 4 Which is not a desired feature of a cemented femoral stem A. Microtexturing and matte finish B. Polished surface C. Macrotexturing D. Cement centralizer Explanation: Certain design features of cemented stems have become generally accepted. 1. Metal of high modulus of elasticity: The stem should be fabricated of high strength superalloy. Most designers favor cobalt-chrome alloy because its higher modulus of elasticity may reduce stresses within the proximal cement mantle. 2. Rounded edges and collar: The cross section of the stem should have a broad medial border and preferably broader lateral border to load the proximal cement mantle in compression. Sharp edges produce local stress risers that may initiate fracture of the cement mantle and should be avoided. A collar aids in determining the depth of insertion and may diminish resorption of bone in the medial neck. 3. Surface macro texturing: Mounting evidence suggests that failure of cemented stems is initiated at the prosthesis-cement interface with debonding and subsequent cement fracture. Various types of surface macro texturing can improve the bond at this interface ( grooves and ridges). 4. Polished surface: There is concern that even with surface modifications the stem may not remain bonded to the cement. If debonding does occur, a stem with a roughened matte surface generates more debris with motion than a stem with a smooth, polished surface. Higher rates of loosening and bone resorption were found with the use of an Exeter stem with a matte surface than with an identical stem with a polished surface. 5. No PMMA coating: The practice of precoating the stem with polymethyl methacrylate (PMMA) has been associated with a higher than normal failure rate with some stem designs and has largely been abandoned. 6. Centralizer: Neutral stem placement within the canal lessens the chance of localized areas of thin cement mantle, which may become fragmented and cause loosening of the stem. Some designs have preformed PMMA centralizers that are affixed to the distal or proximal aspects, or both, of the stem before implantation to centralize the stem within the femoral canal and provide a more uniform cement mantle. The centralizers bond to the new cement and are incorporated into the cement mantle. Q. 5 True about dual mobility hip is all except A. It is an unconstrained tripolar design B. Two areas of articulation share different center of motion to increase the range of motion C. Head –neck ratio is increased to increase stability and range of motion D. Range is increased without increasing the impingement Explanation: A dual mobility acetabular component is an unconstrained tripolar design. The implant consists of a porous coated metal shell with a polished interior that accepts a large polyethylene ball into which a smaller metal or ceramic head is inserted. The two areas of articulation share the same motion center. The design effectively increases the head size and the head-neck ratio of the construct. Implant impingement is reduced and stability is improved without reducing the range of motion as with constrained implants. With dual mobility systems, in vitro motion preferentially occurs at the inner bearing and the outer bearing engages at the extremes of motion. Intraprosthetic dislocations between the small head and polyethylene ball is a complication specific to dual mobility hip joints. Tripolar prosthesis: The tripolar-style mechanism features a small inner bipolar bearing that articulates with an outer true liner. The bipolar segment is larger than the introitus (opening) of the outer liner, preventing dislocation. This is a constrained design, which consists of a bipolar component locked into an outer polyethylene liner during the manufacturing process. The opening of the liner has an embedded metallic locking ring. The bipolar component consists of a 22 mm, 28 mm, or 32 mm prosthetic head that snaps into a polyethylene shell with a polished cobalt-chrome backing. It is free to rotate, but is locked in place by a second inner retaining ring. Indications for constrained liners include insufficient soft tissues, deficient hip abductors, neuromuscular disease, and hips with recurrent dislocation despite well-positioned implants. Constrained acetabular liners have reduced range of motion compared with conventional inserts. Consequently, they are more prone to failure because of prosthetic impingement Both constrained acetabular devices (tripolar prosthesis, dual mobility components cannot be relied on to compensate for technical errors in implant positioning. Dual mobility hip Tripolar hip Constraint Unconstrained design Constrained design Center of motion between two same same articulation Second polythene head Not metal backed Metal backed ROM Increased Reduced Impingement No Yes (between femoral neck and cup) Failure rate low High Tripolar design with a constrained ring. See second head is metal backed. Dual mobility (un constrained) design. See second head is not metal backed. Q. 6 True about rotator interval and its structures is A. Rotator crescent is thick bundle of fibers found at the avascular zone of the coracohumeral ligament running perpendicular to the supraspinatous fibers and spanning the insertions of the supra- and infraspinatus tendons B. Rotator cable is the thin, crescent-shaped sheet of rotator cuff comprising the distal portions of the supraspinatus and infraspinatus insertions C. Intraarticular part of long head of biceps is extrasynovial D. CHL and SGHL are main stabilizers of extraarticular part of long head of biceps Explanation: Rotator interval: It is described as a triangular-shaped space instead of a two-dimensional ‘area’, whose borders are drawn superiorly by the supraspinatus, inferiorly by the subscapularis tendon and by the joint capsule as a ceiling. The floor of this space is the articular surface of the humeral head. It is strengthened by the CHL laterally and by the superior glenohumeral ligament (SGHL) medially. The long head of the biceps tendon lies on the base of the RI space and is covered by a fibrous sheet of capsule. The coracoid process constitutes the medial edge of this triangular space. Microscopically, the RI consists of four layers, as described by Jost et al.The first layer consists of superficial CHL fibres which originate from the coracoid process and insert into the greater and lesser tuberosities following the supraspinatus and subscapularis tendons, respectively. The second layer is a blend of the CHL and rotator cuff tendons. The third layer consists of deep CHL fibres and the fourth layer is a mesh of the SGHL and capsule. The rotator interval capsule is the anterosuperior aspect of the glenohumeral joint capsule, which merges with the CHL and SGHL insertions medial and lateral to the bicipital groove, maintaining the biceps tendon within the groove.