1. General Principles & Basic Science MCQs

The working distance is defined as the distance between the 2 screws closest to the fracture. Decreasing the working distance increases the stiffness of the plate fixation construct. An example of the working distance is provided in Illustrations A and B from Hak's review article. Changing the screw position from A to B results in a less rigid construct that is more suitable for secondary bone healing.Stoffel et al review the biomechanics of locking bridge plate constructs. The working distance is the most important determinant of axial stiffness and torsional rigidity.Decreasing the distance from the plate to the bone, using a longer plate, and increasing the number of screws used also increased stiffness.Egol et al reviews and compares the biomechanics of locked plates and conventional nonlocked plates. Locked plates are most indicated for diaphyseal- metaphyseal junction fractures in osteoporotic bone, severely comminuted fractures, indirect fracture reduction, and fractures where anatomical constraints prevent plating on the tension side of the bone. Conventional nonlocked plates are the fixation of choice for periarticularfractures that require anatomic reduction, and nonunions that require compression to enhance healing.A 47-year-old man complains of long standing pain involving the right index, middle, and ring fingers. A clinical image is shown in Figure A. A radiograph is provided in Figure B. Which of the following is the most likely diagnosis?GoutOsteoarthritisRheumatoid arthritisSeptic arthritisPsoriatic arthritisThe clinical presentation and radiograph are consistent with psoriatic arthritis. Figure A shows a swollen "sausage digit" (dactylitis) and nail pitting (onychodystrophy)characteristic of this condition. Figure B demonstrates the classic "pencil-in-cup" radiographic deformity seen in DIP arthritis, a common orthopaedic manifestation of psoriatic arthritis. Psoriatic arthritis affects 5 to10% of patients with psoriasis of the skin. However, the spectrum ofsymptoms varies greatly from mild and self-limiting to destructive arthritis. It most commonly affects the hands and feet, but can also involve the spine and sacroiliac joints. Primary treatment is medicinal with NSAIDS, methotrexate, and TNF-alpha inhibitors.High infection rates have been reported with surgical intervention. Illustration A is an closer image depicting psoriatic onychodystrophyis. Illustration B illustrates a "pencil-in- cup" deformity.Which of the following study designs represent a level III evidence study?Prospective, randomized controlled trialRetrospective case-control studyRetrospective case seriesProspective cohort studyExpert opinionThe practice of evidence based medicine means integrating individual clinical expertise with the best available external clinical evidence from systematic research. Therapeutic study hierarchy of evidence has been established to better analyze studies in a reproducible fashion. Level I studies include well- designed randomized controlled prospective studies (RCT). Level II includelower quality designed prospective RCT as well as prospective cohort studies. Level III include retrospective cohort studies and case-control studies. Level IV include case series. Level V include case reports, expert opinion, and personal observation. This is summarized in illustration A. The referenced article by Brighton et al is a review of how the level of evidence has evolved and how the different levels can carry varied amounts of impact on clinical treatments and future research.A prosthetic polycentric knee with hydraulic swing control is chosen for a very active 63-year-old transfemoral amputee. All of the following appropriately describe the features of this prosthesis EXCEPT:Flexes in a controlled mannerVariable cadenceAbility to walk at a moderately fast paceKnee center of rotation is fixed anterior to the line of weight bearingWeighs more than a constant friction knee that has a manual extension locking mechanismA polycentric knee has a variable, not fixed, center of rotation. When the center of rotation is posterior to the line of weight bearing it allows control in the stance phase, but makes flexion more difficult. However, when the center of rotation is anterior to the line of weight bearing, flexion is improved but control is sacrificed. An example of this prosthesis is shown in illustration A.The piston mechanism in the hydraulic knee allows variable cadence by changing resistance to knee flexion. This prosthesis also flexes in a controlled manner by limiting excessive flexion and by extending earlier in the gait cycle.The polycentric knee with hydraulic swing control is best for active patients who prefer greater utility and variability but it does weigh more than the constant-friction knee hinge that has a manual extension locking mechanism.The review articles by Michael and Friel review the prescription options for lower extremity prostheses.Level 1 evidence has shown vitamin C reduces the incidence of reflex sympathetic dystrophy (RSD) or complex regional pain syndrome type I (CRPS) in patients with which of the following?Tarsal tunnel syndromeDistal radius fracturesCarpal tunnel syndromeCervical radiculopathy from herniated nucleus pulposisAnkle fracturesCorrent answer: 2Two different prospective, double-blind studies performed by the same institution have shown that vitamin C administration is associated with a lower risk of RSD (i.e CRPS) after wrist fractures. Vitamin C is thought to reducelipid peroxidation, scavenge free hydroxyl radicals, protect the capillary endothelium, and inhibit vascular permeability.The first trial by Zollinger was published in Lancet and included 115 adults with 119 fractures treated with conservative management. They found that RSD/CRPS occurred in four (7%) wrists in the vitamin C group (500mg daily for 50 days) and 14 (22%) in the placebo group.The second trial by Zollinger published in JBJS included 317 adult patients sustaining 328 distal radius fractures treated conservatively. They had allocated treatment groups to 200mg, 500mg, or 1500mg vitamin C dosagesfor 50 days. RSD/CRPS occurrence was 4.2% in the 200mg group, 1.8% in the 500mg group, and 1.7% in the 1500mg group and thus the 500mg dosage for50 days was recommended at the conclusion of the study. Patients making early cast- related complaints to their provider had a higher incidence of developing RSD/CRPS.It should also be noted that a recent double blinded randomized controlled trial by Ekrol et al found no statistical significant benefit of Vitamin C on the outcome of distal radius fractures.Which of the following best describes the mechanism by which osteoprotegerin (OPG) plays a role in RANKL-mediated osteoclast bone resorption?inhibits RANKL-mediated osteoclast bone resorption by directly binding to RANKLinhibits RANKL-mediated osteoclast bone resorption by directly binding to the RANK receptor on osteoclastsstimulates RANKL-mediated osteoclast bone resorption by directly binding to RANKLstimulates RANKL-mediated osteoclast bone resorption by directly binding to theRANK receptor on osteoclastsstimulates RANKL-mediated osteoclast bone resorption by directly binding to PTHOsteoclastic bone resorption is the final common mechanism for osteolysis, whether due to a pathologic lytic lesion, macrophage activation in particle wear, or normal remodeling. The RANKL mechanism controls the coupling of osteoblast and osteoclast activation.RANKL is expressed from osteoblasts and bone-marrow stromal cells. When RANKL binds to the RANKL receptor (receptor/activator of NF-[kappa]B) on the cell membrane of osteoclasts) itstimulates differentiation from osteoclast progenitor cells to mature osteoclasts. Mature osteoclasts proceed with osteoclastic bone resporption. Osteoprotegerin (OPG) acts as a decoy receptor by binding to RANKL and blocking the interaction between RANKL and the RANK-receptor and consequently inhibiting osteoclast formation and activation.The reference by Clohisy et al reviews recent developments in our understanding of the cellular and molecular events regulating osteoclast- mediated bone resorption and discusses the role of the RANKL pathway in several disease states, including osteolysis associated with inflammatory arthritis and cancer-induced bone loss.The reference by Goater et al studied the potential of OPG gene therapy by evaluating the ability of transfected synoviocytes expressing OPG to prevent wear debris-induced osteoclastogenesis. They found a decrease in the amount of bone resorption in mice with the transfected OPG gene. The RANKL pathway is shown in Illustration A below and further described in the linked video.Level 1 evidence has shown Low-intensity Pulsed Ultrasound Stimulation (LIPUS) decreased the time to fracture union in all of the the following injuries EXCEPT?Radius shaft fractureDistal radius fractureTibia shaft fracture treated with castingTibia shaft fracture treated with reamed intramedullary nailingScaphoid fractureTibia shaft fractures treated with reamed intramedullary nailing do not have Level 1 evidence supporting adjunctive LIPUS treatment. Low-intensity pulsed ultrasound (LIPUS) "bone stimulators" deliver 30mW/cm2 pulsed-waves via an external device over the fracture site.The meta-analysis by Busse et al found 6 randomized, controlled trials evaluating LIPUS. They concluded that low-intensity pulsed ultrasound treatment may significantly reduce the time to fracture healing for fractures treated nonoperatively.The metanalysis cites that Emami et al found no benefit to LIPUS treatment on intramedullary fixed tibial fractures. Injuries described in the metaanalysis as having positive benefits from LIPUS include radius shaft(Cook et al), distal radius(Kristiansen et al), scaphoid(Mayr et al), and tibia treated with casting (Heckman et al).The Level 1 study by Heckman et al of 67 patients found a significant decrease in the time to clinical healing in tibia fractures treated with casting and no serious complications with its use.A 58-year-old Jehovah's Witness male presents with severe right hip pain due to osteoarthritis. He has failed exhaustive physical therapy, steroid injections, and activity modifications, and now wishes to proceed with a right total hip arthroplasty. During the procedure, there is profound blood loss with associated hypotension. Which of the following is generally the most preferred method for treating the patient's acute intraoperative anemia?Iron supplementationSubcutaneous erythropoietin administrationABO-matched allogeneic blood transfusionContinuous tranexamic acid infusionUse of cell salvageThe patient has experienced a greater than expected blood loss during the procedure and has developed hemodynamic instability as a result. Given that the patient is a Jehovah's Witness, the use of a cell salvage (Cell Saver) is most preferred method for treating the patient's acute blood loss anemia.Signficant intraoperative blood loss is a risk associated with major orthopedic procedures such as joint arthroplasty, and spine, tumor, and trauma surgeries. The most effective method of mitigating this risk is by maintaining good hemostasis during the procedure.Tranexamic acid (TXA), cell saver, and allogeneic blood transfusion are adjunctive modalities to limit and address excessive intraoperative blood loss. Patients who are Jehovah's Witnesses are generally not amenable to allogeneic blood transfusions but can often be transfused with their own blood. The use of intraoperative cell saver allows for the recycling of the patient's own blood that is obtained with suction, and this can then be used later to transfuse the patient. However, this should be discussed with the patient pre- operatively, as some Jehovah's witnesses may be amenable to allogenic blood transfusion or conversely be opposed to cell saver.Moonen et al. reviewed perioperative blood management in elective orthopedic surgery procedures. The authors stated that the gold standard for preventing intraoperative blood loss was by maintaining adequate hemostasis and dissecting through anatomically correct tissue planes. They proposed the useof pre-operative erythropoietin and iron supplementation, pre-operative autologous blood donation, platelet-rich plasmapheresis, hypotensive epidural anesthesia, and intra- operative cell saving as adjunctive blood loss management modalities. The authors concluded that allogenic blood transfusion should be based on physiologic variables, risks of disease transmission, and patient preference.Imai et al. performed a retrospective study of intraoperative and postoperative blood loss in patients undergoing primary total hip arthroplasty that were treated with either a control or TXA at various time points in the perioperative period. They found that patients who received TXA either 10 minutes prior to surgery or 6 hours after the original dose had a significant decrease in periopreative blood loss. Postoperative blood loss was significantly decreasedin all patients that received TXA. The authors concluded that TXA is an effective adjunct for minimizing blood loss during arthroplasty procedures.Incorrect Answers:According to the 2008 National Osteoporosis Foundation Guidelines for Pharmacologic Treatment of Osteoporosis, when are bisphosphonates indicated for the treatment or prevention of osteoporosis?DEXA T-score between -1.0 and -2.5FRAX calculated 10-year hip fracture risk of >3%FRAX calculated 10-year risk of major osteoporosis-related fracture of>10%The 2008 National Osteoporosis Foundation Guidelines for Pharmacologic Treatment of Osteoporosis suggests that pharmacologic treatment should be considered for a DEXA T-score between -1.0 and -2.5 at the femoral neck/spine AND 10-year risk of hip fracture ≥ 3%.Osteoporosis affects more than 12 million Americans per year, with the burden falling heaviest on postmenopausal women. Because of decreased bone strength, patients with osteoporosis are susceptible to fragility fractures. With no additional risk factors, a 65- year-old Caucasian woman has an estimated10% 10-year risk of a fragility fracture. FRAX (World Health Organization Fracture Risk Assessment Tool) calculates 10-year risk of fracture based on the following variables: age, sex, race, height, weight, BMI, history of fragilityfracture, parental history of hip fracture, use of oral glucocorticoids, secondary osteoporosis and alcohol use to calculate 10-year risk of fracture.Unnanuntana et al. discussed the utility of the FRAX tool as an assessment modality for prediction of fracture risk. The authors advocated for treatment with osteopenia (T-score of-1.0 to -2.5) combined with either a ten-year risk of hip fracture >= 3% or a ten-year risk of major osteoporosis-related fractureof >= 20% as calculated by FRAX. They also discussed biochemical markers of bone formation and resorption, which are useful for monitoring the efficacy of antiresorptive therapy and may help identify patients at high risk for fracture.Cosman et al. review the 2008 National Osteoporosis Foundation guidelines and support that pharmacologic treatment for osteoporosis should be considered if patients are postmenopausal women or men > 50 years of age AND meet one of the following criteria: have a prior hip or vertebral fracture, a T score -2.5 or less at the femoral neck or spine, OR a T score between -1.0 and -2.5 at the femoral neck or spine AND a 10-year risk of hip fracturegreater than 3% or 10-year risk of major osteoporosis-related fracture greater than 20%. They conclude that DEXA scans should be repeated every 1-2 years if patients are undergoing pharmacologic treatment.Gass et al. review the epidemiology and tiered management strategy for osteoporosis. They discuss the first line prevention, treatment of secondary causes of osteoporosis, and finally pharmacologic interventions, all in an effort to mitigate fracture risk and the burden that osteoporotic fractures on the health care system.Illustrations:Illustration A outlines the variables taken into account in the FRAX score calculator.Incorrect answers:ADDITIONALLY has either a ten-year risk of hip fracture >= 3% or a ten-year risk of major osteoporosis-related fracture of >= 20% (or both) as calculated by the FRAX tool.>20% as calculated by the FRAX tool in order to meet the criterion set forth in the 2008 National Osteoporosis Foundation guidelines. Combined with documented osteopenia (T- score of -1.0 to -2.5), bisphosphonate therapy would be indicated.>20% as calculated by the FRAX tool in order to meet the criterion set forth in the 2008 National Osteoporosis Foundation guidelines.Which of the following bone graft substitutes has the fastest resorption characteristics?Calcium sulfateTricalcium phosphateHydroxyapatiteFibular allograftCortical iliac crest autograftOf the three bone graft substitutes listed (calcium sulfate, tricalcium phosphate, and hydroxyapatite), calcium sulfate has the fastest resorption characteristics. Fibular allograft and cortical iliac crest autograft are not considered bone graft substitutes.Calcium sulfate, tricalcium phosphate, and hydroxyapatite are all "osteoconductive" bone graft substitutes, meaning that these implants provide a surface and structure that facilitates the attachment, migration, proliferation, differentiation and survival of osteogenic stem and progenitor cells. Each has different chemical, macro- and microstructural properties. Calcium sulfate (plaster of Paris) is a low-molecular weight soluble compound that must be implanted adjacent to viable periosteum to work. It is reabsorbed by aprocess of dissolution over a period of 5-7 weeks.Jamali, et al., found that calcium sulphate was completely reabsorbed by 6 weeks. Tricalcium phosphate has compressive strength similar to cancellous bone, but is brittle and weak under tension and shear. It undergoes reabsorbtion via dissolution and fragmentation over 6-18 months; unfortunately less bone volume is produced than tricalcium phosphate absorbed. For this reason, it is used clinically as an adjunct with other lessabsorbable substitutes.Moore et al discuss that hydroxyapatite forms the principle mineral content of bone. Synthetically, it is available in ceramic and non-ceramic forms as porous or solid, blocks or granules. HA has good compressive strength, but is weak in tension and shear and brittle making it fracture-prone in shock loading. Ceramic HA preparations are resistant to absorption in vivo, which occurs at 1-2% per year. Non-ceramic HA is more readily absorbed.Which of the following techniques increases strength and stability to an external fixation construct?Unicortical pin fixationDecreasing total pin separation distanceIncreased working distance from the pin to fracture siteDecreasing the distance between the bone and the constructUsing smaller diameter pinsThere are several methods that can be used to increase the strength of an external fixation construct. Decreasing the distance from the bar to the bone increases stability and strengthens the construct. Some other methods to increase stability include: good bone- to-bone fracture end apposition, using an increased number of pins, using larger pins, small distance from the near pins to the fracture site (smaller working distance), increased spacing between the near and far pins, and bicortical pin fixation.Tencer et al looked at biomechanical aspects of external fixation systems. They demonstrated that system rigidity could be increased by maximizing pin separation distance in the fracture component and the number of pins used while minimizing pin separation distance across the fracture site and the sidebar offset distance from bone.Incorrect Answers: Answer choices 1,2,3, and 5 all act to decrease external fixation construct strength.A 62-year-old woman with Paget’s disease is started on a non- nitrogen containing bisphosphonate for treatment of her condition.What is the mechanism of action of this drug?Inhibition of farnesyl diphosphate synthaseConversion of drug into a non-functioning ATP-analogueInterference of isoprenylation of small GTPasesInhibition of geranylgeranyl diphosphate synthase (GGPPS)Downregulation of the undecaprenyl diphosphate synthase (UPPS) pathwayBisphosphonates are a class of antiresorptive agents used to treat diseases characterized by osteoclast-mediated bone resorption. Non-nitrogen containing bisphosphonates (such as etidronate) are metabolized into non-functioningATP analogues which cause eventual osteoclast apoptosis. Nitrogen containing bisphsphonates (alendrolate/Fosamax and Zoledronic acid/Zometa) act by inhibiting farnesyl diphosphate synthase (FPPS), resulting in decreased prenylation of small GTPases.Reszka et al reviewed nitrogen containing bisphosphonates. They outlined the mechanism of action on farnesyl diphosphate synthase in the cholesterol biosynthesis pathway.Guo et al also reviewed the mechanism of nitrogen-containing bisphosphonates. In addition to showing the decrease in prenylation of GTPase, they were shown to inhibit geranylgeranyl diphosphate synthase (GGPPS), as well as undecaprenyl diphosphate synthase (UPPS).Morris et al reviewed the bisphosphonates currently approved by the FDA. They outlined their use in the treatment of Paget disease, metastatic bone disease and widening applications in OI and fibrous dysplasia.Incorrect answers:1,3,4,5: Mechanism of nitrogen-containing bisphosphonates.A 58-year-old female falls and sustains the injury shown in Figures A and B. Following surgical treatment of the fracture, which of the following is the most appropriate additional investigation?MRI of the pelvisUrine electrophoresisCT scan of the pelvisBone scanDEXA scanFigures A and B depicts a femoral neck fracture. Medical management of postmenopausal women with fragility fractures (distal radius, femoral neck, vertebral compression fractures) includes dual-energy x-ray absorptiometry (DEXA) testing.Following the diagnosis of osteoporosis, bisphosphonates, calcitonin or other medical treatments may be initiated.Oyen et al examined 1794 patients with fractures of the distal radius. As one- third of the men and half of the women had bone mineral density (BMD) suggesting osteoporosis, they concluded that all patients aged 50 or above should have bone densitometry testing.Freedman et al reviewed 1162 women with distal radius fractures. They determined that the rate of diagnostic workup and medical treatment decreases as patient age increases at the time of fracture.A 52-year old woman who is not on any hormone replacement therapy (HRT) falls from standing height and sustains the injury seen in Figure A. Review of her medical history reveals that she carries a diagnosis of osteoporosis, and that her latest T-score was -3.0. How much calcium should she have been consuming on a daily basis prior to sustaining her injury?

DISCUSSION: Articular cartilage is a highly organized viscoelastic material, and load transmission depends on the specific composition of the extracellular matrix. Articular cartilage is devoid of neural, lymphatic, and blood vessel tissue. The extracellular matrix consists of water, proteoglycans, and collagen. Water comprises most of the wet weight (65% to 80%). Type II collagen comprises 95% of the collagen. The collagen and proteoglycan (keratan sulfate and chondroitin sulfate) matrix and its high water content are responsible for the mechanical properties of the articular cartilage. REFERENCES: Buckwalter JA, Mankin HJ: Articular cartilage: Degeneration and osteoarthritis, repair, regeneration, and transplantation. Inst Course Lect 1998;47:487-504. Koval KJ (ed): Orthopaedic Knowledge Update 7. Rosemont, IL, American Academy of Orthopaedic Surgeons, 2002, pp 3-18.

DISCUSSION: Erb’s palsy is the most common form of obstetrical plexus palsy resulting in C5, C6, or upper trunk deficits. This causes loss of shoulder abduction and elbow flexion. The biceps muscle and the brachialis muscles are predominately responsible for flexion of the elbow. Each of these muscles is innervated by individual branches of the musculocutaneous nerve which are supplied predominately by axons from the C6 nerve root and the upper trunk of the brachial plexus. REFERENCES: Netter F: The Ciba Collection of Medical Illustrations: The Musculoskeletal System, Part 1: Anatomy, Physiology and Metabolic Disorders. West Caldwell, NJ, Ciba-Geigy Corporation, 1987, vol 8, pp 28-29. Wolock B, Millesi H: Brachial plexus-applied anatomy and operative exposure, in Gelberman RH (ed): Operative Nerve Repair and Reconstruction. Philadelphia, PA, JB Lippincott, 1991, pp 1255-1272. Zancolli E: Reconstructive surgery in brachial plexus sequelae, in Gupta A, Kay S, Scheker L (eds): The Growing Hand. London, England, Mosby, 1999, p 807.

Hybrid locked plating refers to the use of nonlocked and locked screws in the same fixation construct. Hybrid plating offers the advantages of both traditional plating and locked plating. Nonlocked screws are inserted first to "lag" the bone to the plate, thereby using the plate as a reduction tool. After fixation with nonlocked screws in both the proximal and distal fragments, locked screws can be added. Locked screws in the distal fragment create a fixed angle device that is resistant to varus collapse. Locked screws in the diaphyseal fragment are indicated when there is associated osteoporosis.

Increased granulation tissue production occurs with prolonged immobilization following contusion injury; this condition may lead to myositis ossificans. Prolonged immobilization can lead to contraction of scar, poor structural organization of the regenerating muscle, and, ultimately, scar tissue. Muscles immobilized following contusion injury were pulled to failure at tensile strengths much lower than tolerated by mobilized limbs. In addition, there is a decrease in tensile stiffness.

Rickets is a disorder of bones in children that results from decreased calcium available in the blood resulting in poor mineralization of bone that can lead to fractures and deformity. The most common cause of rickets is from vitamin D deficiency but it can also be caused by poor nutrition or gastrointestinal disease that results in poor calcium absorption such as celiac disease or severe diarrhea from other causes. Rickets is not primarily a physeal disorder. Osteogenesis imperfecta is caused by a defect in type I collagen. A defect in the ext-1 gene is often seen in patients with multiple hereditary exostoses. Fibrous dysplasia also can result in bone deformity and fractures due to production of dysplastic fibrous bone but is not caused by calcium or vitamin D deficiency.

Trabecular bone is remodeled through osteoclast activation that creates a resorption pit known as a Howship lacuna. After the pit is formed, osteoclasts are replaced by osteoblasts that form new bone matrix. The cement line separates new bone formation from resorption. Cutting cones are created in cortical bone remodeling. Haversian canals carry nerves and blood vessels longitudinally in bone, while Volkmann canals connect different Haversian canals.

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.

OPG is a receptor that competitively binds with RANKL, blocking the interaction with RANK and inhibiting osteoclastogenesis. PTH, secreted by the chief cells of the parathyroid gland, is active in calcium homeostasis independent of inflammatory arthropathies. PTH increases serum calcium indirectly by binding to osteoblasts, increasing expression of RANKL and decreasing expression of OPG. The interaction of RANKL to RANK in turn stimulates osteoclast precursors to fuse, formingosteoclasts to enhance bone resorption. The pannus of rheumatoid arthritis and monosodium urate crystals of gouty tophi have been shown to trigger release of inflammatory cytokines such as IL-6, IL-8 and tumor necrosis factor alpha. The key to osteoclastic bone resorption of inflammatory arthropathy is regulated by the interaction of RANKL, expressed in osteoblasts and activated T cells, and RANK, expressed in osteoclast progenitors and mature osteoclasts. In inflammatory arthropathy, RANKL expression is increased and OPG is reduced, resulting in increased cortical and subchondral bone.

DISCUSSION: Numerous clinical and basic science investigations have evaluated meniscal tear characteristics to identify factors that either promote or mitigate against meniscal healing. Complex tears have been noted to heal poorly, while longitudinal tears heal more predictably. Tear length, time from injury to repair, medial versus lateral meniscal tears, and the use of a fibrin clot have not been shown to consistently affect meniscal healing. However, rim width, the distance of the tear site from the peripheral meniscocapsular junction (vascular supply), has been shown to have a significant role in the ability of a meniscus repair to heal.

The patient has transient osteoporosis of the hip. Transient osteoporosis is usually a self-limited condition that is most frequently seen in women in the third trimester of pregnancy and in men in the sixth decade of life. Transient osteoporosis is best treated with protected weight bearing.

The superficial zone of articular cartilage has the highest water content (80%) and the lowest proteoglycan content. The collagen fibers (mostly Type II) are arranged parallel to the joint surface to resist shear forces. The deep zone has the highest proteoglycan content, the lowest water content, and collagen fibers arranged perpendicular to the joint line to resist compressive forces.

The tidemark is a highly visible basophilic line on histology that separates the uncalcified deep zone of articular cartilage from the calcified cartilage zone. It serves as an important mechanical tethering point where the collagen fibers from the deep zone securely anchor into the calcified layer, and it is a site of significant shear stress. The tidemark advances outward with age, thinning the uncalcified cartilage.

The superficial (tangential) zone of articular cartilage is the thinnest layer but has the highest water content (approx. 80%) and the lowest proteoglycan content. Its collagen fibrils (primarily Type II and IX) are densely packed and oriented parallel to the joint surface to resist shear forces.

Proteoglycan aggregates in cartilage are composed of many aggrecan monomers attached to a central hyaluronic acid (hyaluronan) backbone. This non-covalent binding is inherently unstable but is stabilized by Link protein, which binds to both the aggrecan core protein and the hyaluronan chain.

Trunnionosis (taper corrosion) is exacerbated by mixed metal junctions (e.g., CoCr head on Ti stem), large head diameters, and small/thin trunnions. A large CoCr head increases the lever arm and toggle at the head-neck junction, while a small titanium trunnion offers less surface area for interference fit, significantly increasing the risk of fretting and crevice corrosion.

The direct anterior approach (Smith-Petersen) utilizes the true internervous plane between the Sartorius (femoral nerve) and the Tensor fasciae latae (superior gluteal nerve). The ascending branches of the lateral femoral circumflex artery cross this interval transversely and must be identified and ligated to prevent significant postoperative hematoma.

Members of the Transforming Growth Factor-beta (TGF-β) superfamily, particularly TGF-β1, TGF-β3, and certain Bone Morphogenetic Proteins (BMPs), are the primary chondroinductive factors used in vitro and in tissue engineering to drive undifferentiated mesenchymal stem cells toward a chondrogenic lineage.

The superficial (tangential) zone of articular cartilage is characterized by the highest water content (up to 80%), the lowest concentration of proteoglycans, and densely packed type II collagen fibers oriented parallel to the articular surface to resist shear forces.