FNB MOCK TEST 3 Answers & Explanations Q. 1 What is true about arthritic hip joint? A. The abductor lever arm is lengthened B. the ratio of the lever arm of the body weight to that of the abductors is increased C. The body weight lever arm is lengthened D. The abductor lever arm remains unchanged The abductor lever arm is shortened in arthritis and other hip disorders in which part or all of the head is lost or the neck is shortened. In an arthritic hip, although the length of the body weight lever arm does not change the ratio of the lever arm of the body weight to that of the abductors may be 4 : 1. The lengths of the two lever arms can be surgically changed to make their ratio approach 1 : 1. Theoretically, this reduces the total load on the hip by 30%. Answer: B Q. 2 After total hip arthroplasty which factor will cause more stress shielding in the proximal femur around the stem? A. Stem of low modulus of elasticity B. Titanium stem compared to Cobalt-Chromium (COCR) stem C. Cylindrical distal fitting stem compared to tapered stem D. Small diameter stem Stress transfer to the femur is desirable because it provides a physiologic stimulus for maintaining bone mass and preventing disuse osteoporosis. Factors affecting stress shielding are: -Modulus of elasticity: A decrease in the modulus of elasticity of a stem decreases the stress in the stem and increases stresses to the surrounding bone. This is true of stems made of metals with a lower modulus of elasticity, such as a titanium alloy, if the cross sectional diameter is relatively small. Larger-diameter stems made of the same material are stronger, but they also are stiffer or less elastic, and the increased cross-sectional diameter negates any real benefits of the lower modulus of elasticity. Titaneum stem having lower modulus of elasticity compared to Cobalt-Chromium stems, produces less stress shielding. - Diameter of stem: larger diameter stem produces more stress shielding. Larger-diameter stems made of the same material are stronger, but they also are stiffer or less elastic so less stress is transferred to bone. - Shape of stem: Less stress shielding in tapered stem compared to cylindrical distal geometry stems that filled the diaphysis. Answer: C Q. 3 Not true about large diameter head in THR A. Will increase the ROM B. Will decrease the ROM C. Will increase the jump distance D. Will be more stable for same neck length For a given neck diameter, the use of a larger femoral head increases the head-neck ratio and the range of motion before the neck impinges on the rim of the socket will be greater. When this impingement does occur, the femoral head is levered out of the socket. The “jump distance” is the distance the head must travel to escape the rim of the socket and is generally approximated to be half the diameter of the head. For both of these reasons, a larger-diameter head is theoretically more stable than a smaller one. Answer: B Q. 4 True about cementless stem is A. Type 3 tem is modular B. Type 1 stem is anatomical C. Optimal pore size of porous material for bony ingrowth is below is below 400 μm D. Stress shielding is less in cobalt chromium stem then in titanium stem Cementless femoral stem fixation: Material : Both titanium and cobalt-chromium have been proved to be satisfactory. Titanium has been recommended by many designers because of its superior biocompatibility, high fatigue strength, and lower modulus of elasticity. Bone ingrowth and Porous coating: Variety of surface modifications including porous coatings, grit blasting, plasma spraying, and hydroxyapatite coating have been used to enhance implant fixation. Most experts agree that porous coating should be circumferential at its proximal boundary. The optimal pore size for bone ingrowth into a porous surface to be between 100 and 400 μm. Most porous coated implants currently available have pore sizes in this range. Bone ingrowth into a porous coating provides durable fixation for a cementless stem design. When the stem is of a titanium substrate, the porous surface must be restricted to the bulkier proximal portions of the stem and away from areas that sustain significant tensile stresses, such as on the lateral border of the stem. Bone ongrowth: Bone ongrowth implies growth of bone onto a roughened (albeit nonporous) surface. Ongrowth surfaces are created by grit blasting or plasma spray techniques. Khanduja et al classification system for cementless stems: It is based on the shape of implant. Types 1 through 5 are straight stems, and fixation area increases with type. Type 6 is an anatomic shape. Type 1 Type 2 Type 3 Type 4 Type 5 Type 6 wedging Single- Dual wedge wedge designs stems. Shape They are Tapered in Tapered in Type 5 or Type 6 or flat in the both two planes modular anatomic anteroposteri medial- stems have femoral or plane and lateral and separate components tapered in anteroposteri metaphyseal incorporate the or planes. sleeves and a mediolateral diaphyseal posterior plane segments bow in the that are metaphyseal independentl portion and y sized variably an and anterior instrumente bow in the d. diaphyseal portion, correspondi ng to the geometry of the femoral canal Fixation Cortical Stems Fixation is Extensive fixation engage the achieved ly coated occurs in the proximal more at the implants mediolateral femoral metaphysea with plane only cortex in l- fixation and by three- both diaphyseal along the point mediolateral junction entire fixation and than length of along anteroposteri proximally the stem the length of or planes. as with the stem. types 1 and 2. Canal The femoral Femoral Canal Such prepratio canal is preparation preparatio implants n prepared by typically n requires often are broaching requires distal recommende alone with distal cylindrica d for no distal reaming l reaming patients with reaming followed by and altered broaching of proximal femoral the broaching anatomy, proximal such as femur development al dysplasia. Both stem segments are prepared with reamers, leading to a precise fit with rotational stability obtained both proximally and distally. Type 1- Single wedged proximal fixation Type 2- double wedged proximal fixation Type 3- double wedged but fixation at metaphyseal-diaphyseal junction Type 4- extensively coated, fixation along entire length Type 5- modular Type 6- anatomical Answer: C Q. 5 Concentration of free radicals in HCLPE is reduced by all except A. Remelting B. Annealing C. Vitamin E doping D. Ethylene oxide treatment Higher doses of radiation is used to produce polyethylene with a more highly crosslinked molecular structure (HCLPE). The radiation process also generates uncombined free radicals. If these are allowed to remain, the material is rendered more susceptible to severe oxidative degradation. The concentration of these free radicals can be reduced by a postirradiation heating process, either remelting or annealing. Remelting entails heating the material above its melting point (approximately135° C). Free radicals are virtually eliminated with remelting, but the crystallinity of the resulting material is also reduced. Annealing refers to a process of heating the material just below the melting point. This avoids the reduction in crystallinity and consequent reduction in mechanical properties, but annealing is less effective than remelting in extinguishing residual free radicals. Newer manufacturing methods have sought to mitigate the deleterious effects of remelting. Soaking the radiated polyethylene in vitamin E (or vitamin E “doping”) appears to be effective in scavenging free radicals without a remelting stage. Ethylene oxide and plasma are used for terminal sterilization, not for elimination of free radicals. Answer: D Q. 6 Not true about ceramic on ceramic bearing A. Stripe wear B. Squeaking C. Impingement D. High wear rate Characteristics of ceramic on ceramic bearings 1. Impingement between the femoral neck and rim of the ceramic acetabular component creates problems unique to this type of articulation. 2. Lowest wearing rate: The linear wear rate of alumina-on-alumina has been shown to be 4000 times less than cobalt-chrome alloy-on-polyethylene. 3. Stripe wear: It is specific to ceramic on ceramic bearings. This term describes a long, narrow area of damage resulting from contact between the head and the edge of the ceramic liner. 4. Squeaking/noisy movement: Enthusiasm for ceramic-on-ceramic implants has recently been somewhat tempered by reports of reproducible noise, particularly squeaking. The onset of squeaking usually occurs more than 1 year after implantation, and the development of strip wear has been implicated in noise generation. Incidence up to 10 % has been reported. The etiology of squeaking has not been fully elucidated and is likely multifactorial. Answer: D Q. 7 Which approach was used in original charnley technique of THR A. Posterior B. Anterolateral C. Anterior D. Lateral The original Charnley technique used the anterolateral surgical approach with the patient supine, osteotomy of the greater trochanter, and anterior dislocation of the hip. Answer: B Q. 8 Highest risk to sciatic nerve in THR is in A. DDH B. Coxa vara C. Revision surgery D. AVN Risk of nerve palsy after primary total hip arthroplasty (THA) for arthritis has been found to be 0.5%, for hip dysplasia 2.3%, and 3.5% for revision surgery. Risk factors associated with nerve palsy after total hip arthroplasty include hip dysplasia, posttraumatic arthritis, posterior approach, cementless fixation, revision surgery, female sex, and significant lengthening of the extremity. Answer: C Q. 9 False statement regarding acetabular component position in THR is A. In horizontal cup dislocation occurs posteriorly with hip flexion B. In vertical cup dislocation occurs superiorly with hip adduction C. In excessively anteverted cup anterior dislocation occurs during hip flexion, adduction, and internal rotation.