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1. General Principles & Basic Science MCQs

Practice Set 482 of 789

This practice set contains high-yield board review questions covering key concepts in 1. General Principles & Basic Science. Each clinical scenario is designed to test your diagnostic and management skills relevant to this subspecialty.

Question 9621

Topic: Biology, Genetics & Bone Healing

In an osteoporotic patient, which modification to a standard cortical screw would not significantly improve its pull-out strength?

. Increasing the outer diameter of the screw.
. Using a screw with a larger thread depth (smaller core diameter).
. Using a longer screw to engage more cortices.
. Coating the screw with an osteoconductive material.
. Decreasing the thread pitch (finer threads).

Correct Answer & Explanation

. Increasing the outer diameter of the screw.

Explanation

Osteoporosis means poor bone quality, which is the primary limitation to screw pull-out. While increasing outer diameter (A), thread depth (B), and length (C) can offer some incremental improvement by maximizing engagement of existing bone, these are limited by the bone's inherent weakness. Decreasing the thread pitch (E) means finer threads, which are designed fordense cortical boneand would likely performworsein soft osteoporotic bone where coarser, deeper threads are preferred. An osteoconductive coating (D) could potentially enhance bone ingrowth over time, theoretically improving long-term pull-out, but its immediate impact is less than geometric design changes in the context of initial pull-out strength.

Question 9622

Topic: Biomechanics & Biomaterials

A patient develops a suspected allergic reaction to their internal fixation hardware. Which metal is most commonly implicated in such reactions, leading to the preference for titanium in some cases?

. Aluminum
. Chromium
. Nickel
. Molybdenum
. Vanadium

Correct Answer & Explanation

. Nickel

Explanation

Nickel (C) is a common allergen and a component of stainless steel alloys (e.g., 316L stainless steel). Patients with known nickel allergies may experience skin reactions or local inflammatory responses to stainless steel implants. Titanium and its alloys are generally preferred in such cases as they are highly biocompatible and do not contain nickel. Chromium (B) and Molybdenum (D) are also components of stainless steel but are less commonly implicated in allergic reactions than nickel. Aluminum (A) and Vanadium (E) are used in some titanium alloys but are not common allergens.

Question 9623

Topic: 1. General Principles & Basic Science

What is a potential disadvantage of using self-drilling screws without pre-drilling, especially in dense cortical bone?

. Reduced overall screw purchase compared to conventional screws.
. Increased risk of screw breakage during insertion due to high torque.
. Longer operative time due to slower insertion.
. Inability to create interfragmentary compression.
. Requirement for specialized tapping instruments.

Correct Answer & Explanation

. Increased risk of screw breakage during insertion due to high torque.

Explanation

Self-drilling screws eliminate the separate drilling step. However, in dense cortical bone, the process of drillingandtapping with the screw itself can generate very high torque during insertion. If the torque limit is exceeded, there is an increased risk of breaking the screw, particularly at the fluted tip or at the driver recess. While pull-out strength can sometimes be slightly reduced (A), breakage (B) is a more immediate and significant disadvantage. Operative time (C) is typically reduced. Interfragmentary compression (D) can still be achieved. No tapping instruments (E) are needed, which is an advantage.

Question 9624

Topic: 1. General Principles & Basic Science

Prolonged and excessively rigid screw-plate fixation can lead to 'stress shielding.' What is the primary consequence of stress shielding on bone?

. Increased bone density around the plate due to reactive hypertrophy.
. Delayed union or non-union due to excessive micromotion.
. Osteopenia and weakening of the bone underlying the plate.
. Accelerated resorption of the plate material.
. Development of heterotopic ossification.

Correct Answer & Explanation

. Osteopenia and weakening of the bone underlying the plate.

Explanation

Stress shielding occurs when the implant (e.g., a very rigid plate and screw construct) carries a disproportionately large share of the physiological load, thereby 'shielding' the underlying bone from normal stresses. According to Wolff's Law, bone adapts to the loads placed upon it. If shielded from stress, the bone responds by becoming osteopenic, losing density, and weakening. Increased bone density (A) is the opposite effect. Delayed union (B) is more associated with excessivemotion. Resorption of plate material (D) is not a direct consequence. Heterotopic ossification (E) is unrelated to stress shielding.

Question 9625

Topic: 1. General Principles & Basic Science

During an attempt to remove a screw, the screw head shears off, leaving the shaft embedded in the bone with no exposed purchase point. Which of the following is the least appropriate initial management?

. Attempt to drill a small hole into the center of the shaft and use a reverse extractor.
. Carefully burr around the exposed shaft to create a small purchase point for a grabbing tool.
. Use a screw driver to try and rotate the flush-broken shaft out of the bone.
. Consider using specialized internal vice-grip type extractors if the shaft can be exposed.
. If the fragment is asymptomatic and not interfering, consider leaving it.

Correct Answer & Explanation

. Use a screw driver to try and rotate the flush-broken shaft out of the bone.

Explanation

If the screw head shears off flush or below the bone surface, there is no longer a driver recess to engage. Therefore, attempting to use a screwdriver (C) to rotate the fragment is futile and inappropriate as an initial step. The other options are valid approaches: drilling a pilot hole and using a reverse extractor (A), burring bone to expose the shaft for gripping (B), using specialized extractors (D), or leaving it if asymptomatic (E) (though the question implies an attempt at removal).

Question 9626

Topic: 1. General Principles & Basic Science

Which of the following is not a primary biomechanical characteristic of a bioabsorbable screw compared to a metallic screw?

. Gradual loss of mechanical strength over time.
. No requirement for second surgery for removal.
. Lower initial mechanical strength.
. Higher rate of thermal necrosis during insertion.
. Potential for sterile inflammatory reaction during degradation.

Correct Answer & Explanation

. Higher rate of thermal necrosis during insertion.

Explanation

Bioabsorbable screws (e.g., PLLA) gradually lose mechanical strength as they degrade over time (A) and eventually resorb, eliminating the need for a second surgery (B). They typically have lower initial mechanical strength compared to metallic screws (C). They also have the potential to cause a sterile inflammatory reaction during their degradation process (E) due to acidic byproducts. While any drilling can cause thermal necrosis, bioabsorbable screws themselves do not inherently cause ahigher rateof thermal necrosis during insertion than metallic screws, assuming proper drilling technique and irrigation. The material itself isn't a direct cause of thermal necrosis during insertion, unlike a dull drill bit.

Question 9627

Topic: Biology, Genetics & Bone Healing

Which bone quality characteristic directly contributes to increased screw stripping risk during insertion?

. Increased bone mineral density.
. Presence of a thick cortical layer.
. Poor vascularity.
. Osteoporosis or compromised bone stock.
. High collagen content.

Correct Answer & Explanation

. Osteoporosis or compromised bone stock.

Explanation

Osteoporosis or compromised bone stock (D) significantly increases the risk of screw stripping. In weak or porous bone, the threads cut by the tap or self-tapping screw may not hold effectively, leading to loss of purchase with even moderate torque. Increased bone mineral density (A) and thick cortical layer (B) actuallyreducethe risk of stripping once threads are properly cut, as they provide stronger purchase. Poor vascularity (C) and high collagen content (E) relate to bone healing and elasticity, respectively, but not directly to the mechanical act of stripping during insertion as much as bone density.

Question 9628

Topic: 1. General Principles & Basic Science

When is a self-drilling, self-tapping screw most advantageous?

. In situations where maximum pull-out strength is required in dense cortical bone.
. When meticulous control over thread formation is critical.
. For percutaneous procedures to minimize surgical exposure and steps.
. When precise lag compression across multiple fragments is necessary.
. In revision surgery where bone quality is severely compromised.

Correct Answer & Explanation

. For percutaneous procedures to minimize surgical exposure and steps.

Explanation

Self-drilling, self-tapping screws combine the drilling and tapping steps into one. This significantly reduces the number of instruments and steps required, making them particularly advantageous for percutaneous procedures (C) where surgical exposure is limited and efficiency is key. They reduce operative time and soft tissue disruption. While convenient, they do not necessarily providemaximumpull-out strength (A) compared to carefully pre-drilled and tapped holes, nor do they offer meticulous control over thread formation (B). They can be used for compression, but it's not their unique advantage (D). Bone quality issues (E) might favor different fixation, not necessarily self-drilling.

Question 9629

Topic: 1. General Principles & Basic Science

What is the typical thread profile of a screw designed for maximal purchase in soft, cancellous bone?

. Fine pitch, shallow depth.
. Fine pitch, deep depth.
. Coarse pitch, shallow depth.
. Coarse pitch, deep depth.
. Variable pitch, self-cutting.

Correct Answer & Explanation

. Coarse pitch, deep depth.

Explanation

Screws designed for soft, cancellous bone require deep and coarse threads (D) to maximize the contact area and obtain sufficient purchase in the less dense bone. This allows the screw to effectively grip and compact the cancellous bone. Fine pitch and shallow depth (A) are characteristic of cortical screws designed for dense bone. Other options represent less optimal or general descriptions.

Question 9630

Topic: Biology, Genetics & Bone Healing

A 68-year-old female on long-term alendronate for osteoporosis presents with 3 months of progressive dull ache in her left thigh, exacerbated by walking. Radiographs show a unilateral, transverse cortical thickening on the lateral aspect of the subtrochanteric femur, with a small radiolucent line visible on the tension side. She denies trauma. What is the most appropriate initial management step?

. Immediate discontinuation of alendronate and observation.
. Prophylactic intramedullary nailing of the affected femur.
. CT scan to rule out metastatic disease.
. Biopsy of the lesion to confirm diagnosis.
. Switch alendronate to teriparatide and monitor.

Correct Answer & Explanation

. Prophylactic intramedullary nailing of the affected femur.

Explanation

The clinical presentation (prodromal pain, location, long-term bisphosphonate use) and radiological findings (transverse cortical thickening, radiolucent line on tension side) are highly suggestive of an incomplete atypical femoral fracture (AFF). Current guidelines recommend prophylactic intramedullary nailing for symptomatic incomplete AFFs, especially if the cortical breach is evident, to prevent a complete fracture which carries high morbidity. Discontinuation of bisphosphonates is indicated but not the primary management for an impending fracture. A CT scan might be done but is not the most appropriate initial management step for the impending fracture itself once the diagnosis is strongly suspected clinically and radiographically. Biopsy is not typically needed as the diagnosis is clinical and radiographic. Switching to teriparatide might aid bone healing but does not address the immediate risk of fracture.

Question 9631

Topic: Biology, Genetics & Bone Healing

A 5-year-old child presents with multiple long bone fractures since infancy, blue sclera, and hearing loss. Radiographs show generalized osteopenia and 'popcorn' calcifications at the metaphyses. Genetic testing confirms Osteogenesis Imperfecta (OI) Type I. Beyond fracture management, what is the most important long-term therapeutic intervention to improve bone strength and reduce fracture rates in this child?

. Growth hormone therapy.
. Oral calcium and Vitamin D supplementation.
. Intravenous bisphosphonate therapy (e.g., pamidronate).
. Regular strenuous weight-bearing exercise.
. Surgical limb lengthening.

Correct Answer & Explanation

. Intravenous bisphosphonate therapy (e.g., pamidronate).

Explanation

The clinical presentation is classic for Osteogenesis Imperfecta (OI). While good nutrition (including calcium and vitamin D) and gentle physical activity are important, the most effective medical therapy to increase bone mineral density, reduce fracture frequency, and improve mobility in children with OI is intravenous bisphosphonate therapy (C), such as pamidronate or zoledronic acid. Growth hormone (A) is not indicated for OI. Oral calcium and Vitamin D (B) are supportive but not sufficient as primary treatment for severe osteopenia. Strenuous weight-bearing exercise (D) can be risky due to fracture risk and is not the primary intervention for bone strength itself. Surgical limb lengthening (E) is a reconstructive procedure for deformity, not a primary treatment for bone fragility.

Question 9632

Topic: Biology, Genetics & Bone Healing

A 70-year-old male presents with increasing left thigh pain, warmth, and bowing of the left femur. Radiographs show cortical thickening, bone enlargement, and a 'V-shaped' lytic lesion in the subtrochanteric region. Alkaline phosphatase levels are significantly elevated. A bone scan shows increased uptake in the left femur. What is the most appropriate management for this symptomatic patient with active Paget's disease?

. Immediate prophylactic intramedullary nailing.
. High-dose NSAIDs and observation.
. Oral bisphosphonates (e.g., alendronate).
. Intravenous zoledronic acid.
. Calcium and Vitamin D supplementation.

Correct Answer & Explanation

. Intravenous zoledronic acid.

Explanation

This patient has symptomatic Paget's disease with features highly suggestive of an impending pathological fracture ('V-shaped' lytic lesion in the subtrochanteric region, bowing, pain). Elevated alkaline phosphatase confirms active disease. The most effective treatment for active and symptomatic Paget's disease is intravenous bisphosphonates, particularly zoledronic acid (D), which powerfully suppresses osteoclast activity, normalizes alkaline phosphatase, reduces pain, and can facilitate bone healing. Prophylactic nailing (A) might be considered if the 'V-shaped' lesion is a complete transverse fissure or very large, but the primary medical management targets the underlying disease first. NSAIDs (B) only provide symptomatic relief. Oral bisphosphonates (C) are an option but less potent/rapid than IV forms for acute, severe disease. Calcium and Vitamin D (E) are supportive but not primary treatment for Paget's.

Question 9633

Topic: Biology, Genetics & Bone Healing

A 4-month-old infant presents with recurrent fractures, poor weight gain, and frontal bossing. Laboratory investigations reveal very low serum alkaline phosphatase (ALP) levels, elevated plasma pyridoxal-5'-phosphate (PLP), and normal serum calcium and phosphate. What is the most likely diagnosis, and what is the current targeted therapeutic approach?

. Rickets due to Vitamin D deficiency; high-dose oral Vitamin D supplementation.
. Osteogenesis Imperfecta Type II; bisphosphonate therapy.
. Hypophosphatasia; enzyme replacement therapy with asfotase alfa.
. Achondroplasia; limb lengthening procedures.
. X-linked hypophosphatemia; oral phosphate and active Vitamin D analogues.

Correct Answer & Explanation

. Hypophosphatasia; enzyme replacement therapy with asfotase alfa.

Explanation

The combination of recurrent fractures, poor weight gain (failure to thrive), frontal bossing, remarkably low serum alkaline phosphatase (ALP) levels, and elevated plasma pyridoxal-5'-phosphate (PLP) is pathognomonic for Hypophosphatasia (HPP). HPP is a rare genetic disorder characterized by defective bone mineralization due to a deficiency of tissue-nonspecific alkaline phosphatase (TNAP). The current targeted therapy for HPP is enzyme replacement therapy with asfotase alfa, which provides recombinant human TNAP. Rickets (Option A) would typically show low phosphate and/or high ALP depending on type. Osteogenesis Imperfecta (Option B) is a collagen disorder, not primarily an ALP deficiency. Achondroplasia (Option D) is a dwarfism disorder, not typically associated with low ALP or recurrent fractures in infancy. X-linked hypophosphatemia (Option E) presents with low phosphate and rickets, but not low ALP.

Question 9634

Topic: Infection, Pharmacology & VTE

A 68-year-old male undergoes total knee arthroplasty and develops signs of infection 3 months post-op. Aspiration and biopsy confirm a prosthetic joint infection (PJI) caused by Methicillin-Sensitive Staphylococcus aureus (MSSA). What is the most appropriate management strategy?

. Chronic oral antibiotic suppression with rifampin and ciprofloxacin.
. Irrigation and debridement (I&D) with polyethylene exchange, followed by 6 weeks of IV antibiotics (e.g., cefazolin) and 3 months of oral rifampin/ciprofloxacin.
. Staged revision arthroplasty (explantation, antibiotic spacer, reimplantation) with a 6-week course of IV antibiotics.
. Simple aspiration and intra-articular antibiotic injection.
. Lifelong oral antibiotic therapy with a single agent.

Correct Answer & Explanation

. Irrigation and debridement (I&D) with polyethylene exchange, followed by 6 weeks of IV antibiotics (e.g., cefazolin) and 3 months of oral rifampin/ciprofloxacin.

Explanation

For acute prosthetic joint infection (PJI), defined as occurring within 3 months of surgery or an acute hematogenous infection on a well-fixed prosthesis, caused by a susceptible organism like Methicillin-Sensitive Staphylococcus aureus (MSSA), irrigation and debridement (I&D) with polyethylene exchange (DAIR - Debridement, Antibiotics, and Implant Retention) is the treatment of choice, provided the soft tissues are healthy and the implants are stable. This is followed by a prolonged course of intravenous antibiotics (e.g., cefazolin for MSSA) typically for 4-6 weeks, and then a switch to oral antibiotics, often combination therapy including rifampin (due to its excellent biofilm penetration) and a fluoroquinolone (e.g., ciprofloxacin, if susceptible) for a total of 3-6 months. Staged revision (Option C) is reserved for chronic PJI (>3 months post-op), when DAIR fails, or when the organism is resistant. Chronic oral suppression (Option A) is for medically frail patients where surgery is contraindicated. Simple aspiration and injection (Option D) is insufficient. Lifelong single-agent oral antibiotics (Option E) are prone to resistance and less effective.

Question 9635

Topic: Biology, Genetics & Bone Healing
A 45-year-old male presents with recurrent pathological fractures, dental abnormalities (e.g., premature tooth loss), and chronic bone pain. Laboratory evaluation reveals normal calcium, phosphate, PTH, and 25-hydroxyvitamin D levels. Alkaline phosphatase is mildly elevated. Genetic testing confirms mutations in the tissue-nonspecific alkaline phosphatase (TNSALP) gene. Which of the following is the most definitive treatment for this patient's underlying condition?
. Bisphosphonates
. Teriparatide
. Enzyme replacement therapy with asfotase alfa
. High-dose vitamin D supplementation
. Phosphate binders

Correct Answer & Explanation

. Enzyme replacement therapy with asfotase alfa

Explanation

The clinical presentation (pathological fractures, dental abnormalities, chronic bone pain, normal Ca/Phos/PTH/Vit D, elevated ALP, TNSALP gene mutation) is classic for adult hypophosphatasia. Asfotase alfa is a recombinant human tissue-nonspecific alkaline phosphatase (TNSALP) that replaces the deficient enzyme, addressing the underlying genetic defect. This enzyme replacement therapy improves bone mineralization, leading to reduced fracture rates and improved muscle strength in affected individuals. Bisphosphonates are contraindicated as they inhibit bone turnover and can worsen mineralization in hypophosphatasia.

Question 9636

Topic: Surgical Anatomy & Approaches

A 35-year-old male sustains a high-energy trauma, resulting in a complex acetabular fracture involving both columns with posterior wall comminution and impaction of the femoral head. A post-reduction CT scan shows persistent articular incongruity of 3 mm and a displaced posterior wall fragment. The patient is neurologically intact. What is the single most critical factor influencing the long-term prognosis after surgical fixation of this injury?

. The timing of surgery (within 24 hours vs. delayed)
. The type of surgical approach used (e.g., ilioinguinal vs. Kocher-Langenbeck)
. The restoration of anatomical articular congruence
. The presence of associated ipsilateral lower extremity fractures
. The need for a blood transfusion during surgery

Correct Answer & Explanation

. The restoration of anatomical articular congruence

Explanation

For acetabular fractures, the most critical determinant of long-term prognosis and prevention of post-traumatic osteoarthritis is the achievement of anatomical reduction and stable fixation, particularly the restoration of articular congruence. Residual articular displacement greater than 1-2 mm significantly increases the risk of developing early degenerative changes and subsequent total hip arthroplasty. While timing, approach, and associated injuries are important, articular congruence directly impacts joint health and survival.

Question 9637

Topic: Surgical Anatomy & Approaches

A 40-year-old male sustains a complete avulsion of the C5-C6 nerve roots from the spinal cord following a motorcycle accident, resulting in a flail shoulder and absent biceps function. He presents 6 months post-injury. What is the most appropriate surgical strategy to restore elbow flexion in this patient?

. Direct nerve repair
. Neurolysis and nerve grafting
. Intercostal nerve transfer to the musculocutaneous nerve
. Triceps motor branch transfer to the anterior division of the axillary nerve
. Tendon transfer (e.g., Steindler flexorplasty)

Correct Answer & Explanation

. Intercostal nerve transfer to the musculocutaneous nerve

Explanation

In cases of complete nerve root avulsion from the spinal cord (a preganglionic injury), direct nerve repair or grafting is impossible due to the lack of a distal stump for the root. Nerve transfers are the reconstructive option of choice. To restore elbow flexion (mediated by the musculocutaneous nerve, which innervates the biceps), a common and effective strategy is to transfer intercostal nerves (typically 3rd and 4th) to the musculocutaneous nerve. The triceps motor branch to axillary nerve transfer is used to restore shoulder abduction/external rotation. Tendon transfers like Steindler flexorplasty are salvage procedures, often considered after nerve repair/transfer failure or when nerve options are exhausted.

Question 9638

Topic: Biology, Genetics & Bone Healing

A 30-year-old male with X-linked hypophosphatemic rickets presents with chronic bone pain, pseudofractures, and progressive lower extremity deformity despite conventional phosphate and calcitriol therapy. Which of the following newer therapeutic agents is most likely to improve his symptoms and reduce pseudofracture burden by targeting the underlying pathophysiology?

. Denosumab.
. Teriparatide.
. Burosumab.
. Romosozumab.
. Alendronate.

Correct Answer & Explanation

. Burosumab.

Explanation

Burosumab is a monoclonal antibody that targets fibroblast growth factor 23 (FGF23), which is overproduced in X-linked hypophosphatemic rickets (XLH). By inhibiting FGF23, Burosumab increases renal phosphate reabsorption and calcitriol production, directly addressing the underlying pathophysiology of XLH. This leads to improved phosphate levels, reduced bone pain, and healing of rickets and pseudofractures, even in patients refractory to conventional therapy. The other agents listed are for osteoporosis or other metabolic bone diseases but do not specifically target FGF23 in XLH.

Question 9639

Topic: 1. General Principles & Basic Science

During an open reduction and internal fixation of a distal femoral fracture, there is a sudden and significant pulsatile bleed from the popliteal fossa, indicating possible popliteal artery injury. What is the most immediate and critical intraoperative step to manage this complication?

. Apply a tourniquet proximally to the thigh.
. Pack the wound tightly with surgical sponges and immediately obtain vascular surgery consultation.
. Attempt suture ligation of the bleeding vessel.
. Administer systemic tranexamic acid.
. Proceed with fracture fixation to stabilize the bone, then address the bleeding.

Correct Answer & Explanation

. Pack the wound tightly with surgical sponges and immediately obtain vascular surgery consultation.

Explanation

In the event of a significant intraoperative arterial bleed (like from the popliteal artery), the immediate and critical step is to obtain temporary hemostasis. This is most safely and effectively achieved by applying direct pressure or packing the wound tightly with surgical sponges. Simultaneously, vascular surgery consultation must be urgently obtained, as repair of major vessels requires specialized expertise. Blind clamping or suturing can cause further damage, and a tourniquet, while providing temporary control, has its own risks (e.g., worsening ischemia, nerve damage) and should only be used temporarily to gain control, not as a definitive measure. Stabilizing the bone before addressing the bleed is incorrect and could lead to exsanguination.

Question 9640

Topic: Biology, Genetics & Bone Healing

A 68-year-old female presents with a 6-month history of insidious onset left anterior thigh pain. She has been on alendronate for osteoporosis for 10 years. Radiographs show focal lateral cortical thickening in the subtrochanteric region of the left femur, with a short transverse fracture line extending medially. No acute trauma is reported. Which of the following is the most appropriate initial management step?

. Immediate operative fixation with an intramedullary nail.
. Discontinue alendronate, initiate teriparatide, and monitor with serial radiographs.
. Discontinue alendronate, supplement with calcium and Vitamin D, and consider prophylactic contralateral femoral nailing.
. Conservative management with protected weight-bearing and observation, continue alendronate.
. Discontinue alendronate, consider prophylactic ipsilateral intramedullary nailing, and evaluate the contralateral femur.

Correct Answer & Explanation

. Discontinue alendronate, consider prophylactic ipsilateral intramedullary nailing, and evaluate the contralateral femur.

Explanation

This patient presents with a prodromal atypical femoral fracture (AFF) in the left femur, characterized by focal cortical thickening and a transverse fracture line, along with a history of long-term bisphosphonate use. The pain indicates an impending or incomplete fracture. The most appropriate initial management involves discontinuing the bisphosphonate. Prophylactic intramedullary nailing of the affected femur is generally recommended for symptomatic incomplete AFFs due to the high risk of complete fracture. Additionally, the contralateral femur should be evaluated as AFFs can be bilateral (occurring in 50-80% of cases). Teriparatide can be considered for fracture healing, but the immediate concern is mechanical stability. Option A is for complete fractures. Option B and D do not address the mechanical instability adequately. Option C is incomplete as it doesn't mention ipsilateral fixation.

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