I. Total Hip Arthroplasty
A. Overview
1. Total hip arthroplasty (THA) requires complete visualization of the acetabulum and proximal femur.
2. Recognition of the surrounding landmarks is crucial for the correct orientation and implantation of prosthetic components.
3. The ultimate goal of THA is to achieve adequate surgical exposure while minimizing complications.
B. Surgical approaches—The most common surgical approaches for THA, along with their corresponding internervous intervals, major structures at risk, and advantages and disadvantages/risks, are shown in
Table 1.
II. Implant Fixation
A. Cemented THA
1. Cemented femoral components
a. There are two philosophies of cemented femoral fixation. One is based on surface properties to increase implant-to-cement adhesion; the other relies on implant shape.
i. Surface properties—Improved implant-to-cement adhesion is provided by increased surface roughness (Ra value), precoating, or macroscopic grooves or channels. Cement/implant bonding failure has a lower probability of occurring with a rough-surfaced implant. Debonded components will produce subsidence, motion, and wear debris. One disadvantage of rough-surfaced components is that a rough surface produces more wear debris than a smooth one.
*Michael L. Parks, MD, or the department with which he is affiliated has received research or institutional support, miscellaneous nonincome support, commercially derived honoraria, or other nonresearch-related funding from Zimmer, holds stock or stock options in Zimmer, and is a consultant for or an employee of Zimmer.
ii. Implant shape—With this approach, stability is derived from the implant shape. Successful smooth stems have a straight-taper design that allows subsidence. The wedge-shaped implant can subside into the cement mantle, increasing resistance. One advantage of smooth stems is that they generate less debris.
b. Cement techniques
i. First-generation femoral cement techniques: cement mixed by hand in an open bowl; cement placed in canal by hand; no canal lavage or drying; pressure provided by surgeon's thumb
ii. Second-generation techniques: plug, injecting doughy cement, cement gun
iii. Third-generation techniques: porosity reduction, pressurization, pulsatile lavage
c. Clinical study results
i. Results from selected clinical studies on the use of cemented femoral components in THA are shown in
Table 2.
ii. To date, long-term survivorship of cemented femoral components has been excellent.
iii. Fatigue fractures (cracks between preexisting pores in the cement mantle) are the primary mode of component failure.
2. Cemented acetabular components
a. The relatively high failure rate associated with cemented acetabular components has led most US orthopaedic surgeons to use cementless implants.
b. Cemented acetabular components are commonly used for cost containment in low-demand and older (age >60 years) patients.
B. Cementless THA
1. Cementless femoral components—In recent years, surgeons in North America have shifted toward the use of cementless femoral components.
[Table 1. Surgical Approaches for Total Hip Arthroplasty]
[Table 2. Cemented Femoral Components in Total Hip Arthroplasty: Clinical Study Results]
a. Design features/implant shape—Stem designs include tapered, cylindrical, and anatomic.
i. Tapered stems have a proximal-to-distal taper that is designed to interlock in the metaphysis with no diaphyseal fixation. Proximal porous coating or plasma spray macro-texturing is used to impart stability and allow for bone ingrowth. The implant is usually collarless, which allows the prosthesis to be wedged into the bony metaphysis, providing for optimal fit and bone ingrowth. The tapered design allows subsidence into a tight fit and optimizes proximal load sharing of the implant, thereby optimizing bone ingrowth and minimizing stress shielding.
ii. Cylindrical stems usually have a circumferential porous coating. Proximal and distal coating optimizes the surface area for maximum bone ingrowth. Initial stability is dependent on a tight diaphyseal fit. The tubular diaphysis can be reproducibly machined to allow bone ingrowth and a tight fit.
iii. Anatomic stems fill the metaphyseal region in both the coronal and sagittal planes. Adequate fill of the metaphyseal region in both the coronal and sagittal plains is crucial. There is little advantage to matching the implant shape to the anatomy of the femur; high rates of thigh pain have been reported.
b. Clinical study results—Results from selected clinical studies on the use of cementless femoral components in THA are shown in
Table 3.
2. Cementless acetabular components
|
a. |
Long-term studies showed mixed results depending on ingrowth surface. |
[Table 3. Cementless Femoral Components in Total Hip Arthroplasty: Clinical Study Results]
|
b. |
Critical factors for success i. Bone ingrowth or ongrowth ii. Acetabular surface receptive to bone growth (pore size 100 to 400 μm) iii. Micromotion <25 to 50 μm |
|
c. |
Clinical study results i. Cementless acetabular components have improved fixation rates in younger patients (age <60 years). ii. Osteolysis is the major reason for revision (range: 2% to 56%). |
|
d. |
A number of studies have demonstrated excellent fixation at the acetabular component, but revisions have been necessary for polyethylene wear and osteolysis. |
C. Highly porous metals
1. Porous metal constructs that permit ingrowth of human bone may represent a significant advance in reconstructive hip surgery.
2. Both titanium and tantalum are being used.
3. The overall structural and mechanical properties of porous metal mimic dense cancellous bone. The unique geometry of porous metal also mimics cancellous bone, and it is favorable to osteon formation.
4. Compared to other available surface coatings, highly porous metal offers the potential advantage of stronger and faster attachment to healthy underlying bone. However, long-term data are necessary.
III. Hemiarthroplasty of the Hip
A. Indications
1. Hemiarthroplasty is most commonly used to treat displaced femoral neck fractures.
2. It is rarely used to treat osteoarthritis of the hip in younger patients; acetabular erosion is a problem.
3. Hemiarthroplasty can also be used as treatment for femoral head osteonecrosis to preserve acetabular bone stock.
4. It is rarely useful as a salvage procedure when there is inadequate bone stock to allow fixation of a stable acetabular component.
B. Contraindications
1. Inflammatory arthritis
2. Preexisting disease of the acetabulum
3. Sepsis
C. Advantages
1. Hemiarthroplasty is useful for frail, elderly patients with hip fractures.
2. It provides greater range of motion than standard THA.
3. It is also associated with a lower rate of dislocation than THA.
D. Disadvantages
1. Hemiarthroplasty is associated with more wear debris because components are constructed of thinner polyethylene material.
2. Acetabular cartilage wear and erosion may require conversion to THA.
E. Clinical study results
1. In clinical studies, most conversions of hemiarthroplasty to THA occurred because of some combination of loosening of the femoral stem and erosion of the acetabulum.
2. In clinical studies, up to 37% of younger patients (age <50 years) with osteoarthritis who underwent hemiarthroplasty required THA within 2 years because of degeneration of the acetabular cartilage.
3. There was no clear difference at follow-up between unipolar and bipolar bearings for elderly patients with displaced femoral neck fractures.
IV. Hip Resurfacing
A. Indications
1. Hip resurfacing is limited to patients with advanced arthrosis of the hip joint and well-preserved proximal femoral bone. Patients who undergo hip resurfacing are generally younger. Better results have been reported for patients with osteoarthritis than for patients with dysplasia or osteonecrosis.
2. Amstutz and associates described three types of patients for whom hip resurfacing (rather than standard THA) is indicated:
a. Patients with a proximal femoral deformity that makes a standard hip replacement prosthesis difficult to place
b. Patients with a high risk of sepsis because of prior infection or immunosuppression
c. Patients with a neuromuscular disorder (large-diameter component lessens dislocation risk)
B. Contraindications
1. Loss of bone in the femoral head
2. Large femoral neck cysts found at surgery
3. Small or bone-deficient acetabulum
C. Advantages
1. Hip resurfacing preserves bone in the proximal femur.
2. It also provides physiologic stress transfer to the proximal femur.
3. Revision of the femoral resurfacing component is potentially easier than revision of intramedullary THA.
D. Disadvantages
1. Disadvantages of hip resurfacing include a lack of modularity, which reduces the ability to adjust leg length and to correct offset problems.
2. The incidence of postoperative femoral neck fracture ranges from 0% to 4%.
3. Aseptic loosening can occur.
4. Metal debris can elevate metal ion levels in the patient's blood and urine.
5. The best results have been obtained in young males with excellent bone stock in the femoral neck.
E. Clinical study results—Clinical study results for selected studies of metal-on-metal total hip resurfacing are shown in
Table 4. Long-term data are necessary to determine the role of hip resurfacing in young patients.
V. Complications of Total Hip Arthroplasty
A. Heterotopic ossification (HO)
1. The prevalence of small amounts of HO associated with THA has been reported to be as high as 80%.
2. Risk factors for HO include prolonged surgical time, the subtype of osteoarthritis (hypertrophic), and handling of the soft tissues at the time of surgery.
[Table 4. Metal-on-Metal Total Hip Resurfacing: Clinical Study Results]
3. Prophylaxis—Prophylactic treatment for HO includes either oral indomethacin or radiation therapy. Radiation therapy (700 Gy) must be administered within 72 hours after surgery.
B. Vascular injury during screw insertion
1. The incidence of vascular injury during screw insertion is reported to be <1%.
2. Vascular injury during screw placement is less common than nerve injury but is more life-threatening. It may result in significant catastrophic hypotension requiring immediate surgical attention.
3. Vascular anatomy
a. The external iliac artery and vein run along the medial border of the psoas muscle.
b. Wasielewski proposed the hip quadrant system as a guide for safe insertion of screws (
Figure 1). Injury may occur in the anterior superior quadrant during screw insertion for cup placement.
c. The obturator artery and vein, which traverse the quadrilateral surface of the inner pelvis, may also be injured with screw insertion in the anterior superior quadrant (
Figure 2).
4. Mechanisms of vascular injury
a. Occlusion associated with peripheral vascular disease
b. Direct vascular injury
i. Removal of cement
ii. Insertion of screws (Figure 1)
iii. Penetrating instruments/retractors
C. Nerve injury
1. The incidence of postoperative nerve injury ranges from 0% to 3%.
[Figure 1. The quadrant system for safe insertion of screws is based on screws positioned posterior and superior to a line (Line A) drawn between the anterior superior iliac spine (ASIS) and the ischial tuberosity. This line is then bisected with a perpendicular line (Line B) at its midpoint, forming four quadrants. The shaded portion of the illustration indicates the area that is safe for screw insertion.]
2. The peroneal branch of the sciatic nerve is the most commonly injured nerve.
3. Risk factors
a. Revision hip surgery
b. Congenital hip dislocation
[Figure 2. Schematic diagram showing the location of excessively long screws on the quadrilateral surface of the inner pelvis relative to the iliac arterial system. Screws A and B are near the external iliac artery; their acetabular origins are in the anterior superior quadrant.]
c. Female sex
d. Lengthening of the extremity (>4 cm)
4. Causes
a. Direct trauma
b. Excessive tension
c. Ischemia
d. Compression (hematoma or dislocation)
e. Heat of polymethylmethacrylate polymerization
f. The cause of nerve injury is unknown in 40% of cases.
5. Most patients recover fully unless the nerve is transected or severely damaged.
D. Dislocation
1. The incidence of hip dislocation is 1% to 3%, with 70% occurring within the first month after surgery.
a. Infection is the most common reason for revision arthroplasty of the hip, and dislocation is the second most common reason.
[
Figure 3. Lateral radiograph showing the amount of anteversion estimated by comparing the inclination of the cup to a vertical line drawn perpendicular to the coronal plane of the pelvis.]
b. 75% to 90% of postoperative hip dislocations are posterior dislocations
2. Risk factors
a. Female sex
b. Prior hip surgery (most significant risk factor)
c. Posterior surgical approach
i. Most series report two to three times greater risk with the posterior approach.
ii. Complete capsular closure techniques, including reconstruction of the external rotators and capsular attachments, decrease dislocation rates.
d. Increased femoral offset increases tissue tension and stability, thus decreasing the risk of dislocation.
e. A larger femoral head increases stability.
f. Malpositioning of the components (most important risk factor that is under the surgeon's control)
i. Ideal positioning of the component is 40° ± 10° abduction and 15° ± 10° anteversion (Figure 3).
ii. Optimal positioning of the component and restoration of hip mechanics is the best way to prevent dislocation.
3. Treatment
a. Nonsurgical treatment (usually closed reduction followed by protected ambulation) is successful for 60% to 80% of patients with postoperative hip dislocations.
b. Redislocation occurs in 20% to 30% of patients who have undergone closed reduction for postoperative hip dislocation.
c. If component malpositioning is present soon after hip arthroplasty, immediate revision arthroplasty may be required.
d. Chronic or recurrent dislocations require surgical revision.
E. Venous thromboembolic events
1. Incidence
a. Deep venous thrombosis (DVT) occurs in 45% to 57% of patients who undergo hip arthroplasty without prophylaxis.
b. Pulmonary embolism (PE) occurs in 0.7% to 2% of patients who undergo THA without prophylaxis; 0.1% to 0.4% will be fatal. Ninety percent (90%) of PEs originate in the proximal (popliteal and higher) vessels.
2. Risk factors
a. Venous stasis
b. Vessel wall damage
c. Previous thromboembolic disease
d. Altered blood proteins, protein C resistance, lupus anticoagulant, protein S deficiency, antithrombin III deficiency
e. History of cancer and/or chemotherapy
f. Increasing patient age
g. Obesity
h. Oral contraceptive use
i. Tobacco use
3. Evaluation
a. Signs and symptoms of DVT
i. Swelling of the leg
ii. Positive Homan sign: Not sensitive or specific
iii. No specific signs
iv. 50% to 80% are clinically silent
b. Signs and symptoms of PE (Patients also may exhibit no symptoms at all.)
i. Shortness of breath
ii. Difficulty breathing
iii. Chest pain
iv. Tachycardia
v. Cyanosis
vi. Hemoptysis
vii. Hypotension
viii. Anxiety
c. Diagnostic tests
i. Contrast venography is the gold standard for DVT, but it is invasive.
ii. Venous ultrasound is noninvasive, and it is the diagnostic tool of choice for symptomatic clots.
iii. CT pulmonary angiography is now the diagnostic tool of choice for PE.
iv. Ventilation perfusion scan mismatch allows for the diagnosis of PE.
v. Pulmonary angiography is now rarely performed to confirm the diagnosis of PE.
4. Venous thromboprophylaxis
a. Intraoperative prophylactic measures include decreased surgical time; use of regional anesthesia; and decreased time of flexion, internal rotation, or abduction of the leg.
b. Nonpharmacologic prophylactic measures include early postoperative mobilization and the use of pneumatic leg compression devices. Pneumatic compression devices should be used as adjunctive agents with chemoprophylaxis.
c. Pharmacologic prophylaxis includes:
i. Warfarin (factors II, VII, IX, and X), low-molecular-weight heparin (LMWH, factor Xa inhibitor), and fondaparinux (indirect factor Xa inhibitor) have all been shown to provide effective prophylaxis after THA in randomized controlled clinical trials. In general, in randomized trials, LMWH has been more effective than warfarin in preventing symptomatic DVT; however, the LMWHs are also associated with higher bleeding rates.
ii. The use of aspirin as a sole prophylactic agent in patients undergoing total joint arthroplasty remains controversial. Randomized clinical trials are necessary to determine its efficacy. Aspirin therapy should be combined with sequential compression devices.
F. Osteolysis
1. Etiology
a. Osteolysis associated with hip arthroplasty results from particulate wear debris generated by femoral head articulation with a polyethylene liner (or other bearing replacement surface).
b. The host response to wear particles leads to osteoclast activation and osteolysis.
2. Cellular biology of bone resorption
a. Loose implants are surrounded by a membrane containing fibroblasts, macrophages, and inflammatory mediators (prostaglandin E2, interleukin-1, interleukin-6, tumor necrosis factor-α).
b. Local macrophage response to debris activates the inflammatory cascade. The response is influenced by particle size, composition, and the number of particles.
c. Wear particles 0.5 to 5.0 μm induce a maximal response. Most particles produced in THA are <1 μm.
d. Osteolysis may occur secondary to polyethylene, cement, metal, or ceramic wear debris.
3. Polyethylene wear
a. The wear rate of polyethylene correlates with development of osteolysis. Ultra-high-molecular-weight polyethylene liners wear at a rate of 0.1 to 0.2 mm/year.
b. Polyethylene liner wear rates below 0.1 mm/year are associated with decreased development of osteolysis.
c. Factors affecting conventional polyethylene wear resistance
i. Internal destabilization of the polyethylene: Air-sterilized polyethylene may degrade prematurely due to the presence of free radicals, and long shelf storage results in oxidation and early component failure.
ii. Thickness <6 mm increases wear.
iii. Internal cross-linking of polyethylene chains increases wear resistance. The new highly cross-linked polyethylenes may have reduced wear rates over time.
iv. Malalignment of the implant components or socket can increase wear by increasing stress on the outer rim.
v. Patient factors associated with increased wear rate include young age (age <50 years), male sex, and higher activity levels.
d. Effective joint space
i. THA expands the boundaries of pseudosynovial fluid flow.
ii. Components that are not well fixed to the bone allow fluid to migrate along the length of the prosthesis-bone (or cement-bone) interface, thus allowing access of particulate debris to these areas.
4. Evaluation/diagnostic tests for osteolysis
a. Plain radiographs often underestimate the degree of osteolysis.
b. CT scanning is useful for high-risk patients (young patients or patients with high linear wear rates).
5. Treatment
a. Indications for revision—Surgery is indicated when osteolytic lesions are symptomatic, when there is expansive osteolysis involving the posterosuperior acetabular column or >50% of the cup or an enlarging defect, or when wear-through of the polyethylene liner is imminent.
b. Femoral revision
i. Treatment for loose femoral components depends on the ability of the remaining bone to support distal cementless fixation.
ii. This is best determined once the implant has been removed.
c. Acetabular revisions
i. Cemented cups often loosen before significant bony destruction occurs, allowing for straightforward revision.
ii. Cementless implants often present with expansile osteolysis with a well-fixed implant. Studies have reported success with liner exchange, debridement, and bone grafting with retention of the cup. Contraindications to retention of a well-fixed implant include malpositioning of the component, a poor survivorship record of the implant, or inability to obtain adequate hip stability. If the locking mechanism has failed, a new liner can be cemented in place.
Top Testing Facts
1. Third-generation femoral cement techniques include porosity reduction, pressurization, and pulsatile lavage.
2. Excellent femoral fixation can be obtained with proximally coated tapered stems, extensively porous-coated stems, and cemented fixation.
3. Prophylactic treatment for HO includes either oral indomethacin or radiation therapy. Radiation therapy (700 Gy) must be administered within 72 hours after surgery.
4. Complete capsular closure techniques, including reconstruction of the external rotators and capsular attachments, can decrease dislocation rates associated with the posterior approach.
5. LMWHs activate antithrombin and inhibit factor Xa.
6. Osteolysis associated with hip arthroplasty results from particulate wear debris generated by femoral head articulation with a polyethylene liner (or other bearing replacement surface).
7. The wear rate of polyethylene correlates with development of osteolysis. Ultra-high-molecular-weight polyethylene liners wear at a rate of 0.1 to 0.2 mm/year.
8. Air-sterilized polyethylene may degrade prematurely due to the presence of free radicals, and long shelf storage results in oxidation and early component failure.
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