Hip replacement

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Artificial hip
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Hip replacement
An X-ray showing a left hip (right of image) that has been replaced, with the ball of this ball-and-socket joint replaced by a metal head that is set in the femur and the socket replaced by a cup
Other namesHip arthroplasty
ICD-9-CM81.5181.53
MeSHD019644
MedlinePlus002975

Hip replacement is a

hemiarthroplasty generally only replaces the femoral head. Hip replacement is one of the most common orthopaedic operations, though patient satisfaction varies widely. Approximately 58% of total hip replacements are estimated to last 25 years.[1] The average cost of a total hip replacement in 2012 was $40,364 in the United States, and about $7,700 to $12,000 in most European countries.[2]

Medical uses

Total hip replacement is most commonly used to treat joint failure caused by

juvenile rheumatoid arthritis. The aims of the procedure are pain relief and improvement in hip function. Hip replacement is usually considered only after other therapies, such as physical therapy and pain medications, have failed.[citation needed
]

Risks

Risks and complications in hip replacement are similar to those associated with all joint replacements. They can include infection, dislocation, limb length inequality, loosening, impingement, osteolysis, metal sensitivity, nerve palsy, chronic pain and death. Weight loss surgery before a hip replacement does not appear to change outcomes.[3]

Follow-up assessments are conducted to examine the need for revision surgery. However, a UK study showed that only 3-6% of hip replacements needed a revision. Researchers recommended that routine follow-up may not be needed for up to 10 years. At this point, x-rays should be used to assess the joint, and there should be a clinical assessment of pain and mobility.[4][5]

Edema appears around the hip in the hours or days following the surgery. This swelling is typically at its maximum 7 days after the operation,[6] then decreases and disappears over the course of weeks. Only 5% of patients still have swelling 6 months after the operation.[7]

Dislocation

Dislocated artificial hip
Liner wear, particularly when over 2 mm, increases the risk of dislocation.[8] Liner creep, on the other hand, is normal remoulding.[9]

Dislocation (the ball coming out of the socket) is the most common complication. The most common causes vary by the duration since the surgery.[citation needed]

Hip prosthesis dislocation mostly occurs in the first three months after insertion, mainly because of incomplete scar formation and relaxed soft tissues.[8] It takes eight to twelve weeks for the soft tissues injured or cut during surgery to heal. The chance of this is diminished if less tissue is cut, if the cut tissue is repaired and if large diameter head balls are used.

Dislocations occurring between three months and five years after insertion usually occur due to malposition of the components, or dysfunction of nearby muscles.[8]

Risk factors of late dislocation (after five years) mainly include:[8]

  • Female sex
  • Younger age
  • Previous subluxation without complete dislocation
  • Previous trauma
  • Substantial weight loss
  • Recent onset or progression of dementia or a neurological disorder
  • Malposition of the cup
  • Liner wear, particularly when it allows head movement of more than 2 mm within the cup compared to its original position
  • Prosthesis loosening with migration

Surgeons who perform more operations tend to have fewer dislocations. An anterior approach seems to lower dislocation rates when small diameter heads are used, but that benefit has not been shown when compared to modern posterior incisions with the use of larger diameter heads. The use of larger diameter head size in itself decreases dislocation risk, even though this correlation is only found in head sizes up to 28 mm: larger heads do not result in a statistically significant decrease in dislocation rate.[10] Keeping the leg out of certain positions during the first few months after surgery further reduces risk.[citation needed]

Infection

Infection is one of the most common causes for revision of a total hip replacement. The incidence of infection in primary hip replacement is 1% or less in the United States.[11] Risk factors for infection include obesity, diabetes, smoking, immunosuppressive medications or diseases, and history of infection.[citation needed]

In revision surgery, infected tissue surrounding the joint is removed, and the artificial joint replaced. Typically, this is carried out in 2 stages: infected tissue and all joint replacement implants are removed in the first stage, and, after the infection is completely cleared, a new artificial joint is inserted in the second stage. One-stage surgery is also available whereby infected tissue and implants are removed, and the new joint inserted, in a single procedure. One-stage hip revisions were found to be as effective as two-stage procedures at relieving pain and improving hip stiffness and function. One-stage procedures were also better value for money.[12][13]

Limb length inequality

Most adults have a limb length inequality of 0–2 cm which causes no deficits.[14] It is common for people to sense a larger limb length inequality after total hip replacement.[15] Sometimes the leg seems long immediately after surgery when in fact both are equal length. An arthritic hip can develop contractures that make the leg behave as if it is short. When these are relieved with replacement surgery and normal motion and function are restored, the body feels that the limb is now longer than it was. This feeling usually subsides by six months after surgery as the body adjusts to the new hip joint. The cause of this feeling is variable, and usually related to abductor muscle weakness, pelvic obliquity, and minor lengthening of the hip during surgery (<1 cm) to achieve stability and restore the joint to pre-arthritic mechanics. If the limb length difference remains bothersome to the patient more than six months after surgery, a shoe lift can be used. Only in extreme cases is surgery required for correction.[citation needed] The perceived difference in limb length for a patient after surgery is a common cause for lawsuits against the healthcare provider.[16][17][18][19][20]

Fracture

Intraoperative acetabular fracture

Intraoperative fractures may occur. After surgery, bones with internal fixation devices in situ are at risk of periprosthetic fractures at the end of the implant, an area of relative mechanical stress. Post-operative femoral fractures are graded by the Vancouver classification.

Vein thrombosis

heparins and rivaroxaban.[24][25] However, aspirin may not be appropriate in all cases, especially for patients who have additional risk factors for venous thromboembolisms or may have an inadequate response to aspirin.[26]

Some physicians and patients may consider having an

ultrasonography for deep vein thrombosis after hip replacement.[27] However, this kind of screening should only be done when indicated because to perform it routinely would be unnecessary health care.[27]

Intermittent pneumatic compression (IPC) devices are sometimes used for prevention of blood clots following total hip replacement.[28]

Osteolysis

Many long-term problems with hip replacements are the result of osteolysis. This is the loss of bone caused by the body's reaction to polyethylene wear debris, fine bits of plastic that wear off the cup liner over time. An inflammatory process causes bone resorption that may lead to subsequent loosening of the hip implants and even fractures in the bone around the implants. Ceramic bearing surfaces may eliminate the generation of wear particles. Metal cup liners joined with metal heads (metal-on-metal hip arthroplasty) were developed for similar reasons. In the lab these show excellent wear characteristics and benefit from a different mode of lubrication.

Highly cross-linked polyethylene plastic liners experience significantly reduced plastic wear debris. The newer ceramic and metal prostheses may not have long-term performance records. Ceramic piece breakage can lead to catastrophic failure. This occurs in about 2% of implants. They may also cause an audible, high pitched squeaking noise with activity. Metal-on-metal arthroplasty can release metal debris into the body. Highly cross linked polyethylene is not as strong as regular polyethylene. These plastic liners can crack or break free of the metal shell that holds them.[citation needed]

Loosening

Hip prosthesis displaying aseptic loosening (arrows)
Hip prosthesis zones according to DeLee and Charnley,[29] and Gruen.[30] These are used to describe the location of for example areas of loosening.

On radiography, it is normal to see thin radiolucent areas of less than 2 mm around hip prosthesis components, or between a cement mantle and bone. These may indicate loosening of the prosthesis if they are new or changing, while areas greater than 2 mm may be harmless if they are stable.[31] The most important prognostic factors of cemented cups are absence of radiolucent lines in DeLee and Charnley zone I, as well as adequate cement mantle thickness.[32] In the first year after insertion of uncemented femoral stems, it is normal to have mild subsidence (less than 10 mm).[31] The direct anterior approach has been shown to itself be a risk factor for early femoral component loosening.[33][34][35]

Metal sensitivity

Concerns were raised in the early 2000s regarding metal sensitivity and the potential dangers of metal particulate debris from hip prostheses, including the development of pseudotumors, soft tissue masses containing necrotic tissue, around the hip joint. It appears these masses were more common in women, and these patients showed a higher level of iron in the blood. The cause was then unknown, and was probably multifactorial. There may have been a toxic reaction to an excess of particulate metal wear debris or a hypersensitivity reaction to a "normal" amount of metal debris.[36][37]

Metal hypersensitivity is a well-established phenomenon and is not uncommon, affecting about 10–15% of the population.

eczema, redness and itching. Although little is known about the short- and long-term pharmacodynamics and bioavailability of circulating metal degradation products in vivo, there have been many reports of immunologic-type responses temporally associated with implantation of metal components. Individual case reports link immune hypersensitivity reactions with adverse performance of metallic cardiovascular, orthopedic and plastic surgical and dental implants.[38]

Metal toxicity

Most hip replacements consist of cobalt and chromium alloys, or titanium.

cobalt toxicity with hip replacement, particularly metal-on-metal hip replacements, which are no longer in use.[39][40]

Use of metal-on-metal hip replacements from the 1970s was discontinued in the 1980s and 1990s, particularly after the discovery of aseptic lymphocyte-dominant vasculitis-associated lesions (ALVAL). However, the FDA's 510k approval process allowed companies to have new and "improved" metal-on-metal hips approved without much clinical testing.[41] Some people with these prostheses experienced similar reactions to the metal debris as occurred in the 20th century; some devices were recalled.[42][43]

Nerve palsy

Post operative sciatic nerve palsy is another possible complication. The frequency of this complication is low. Femoral nerve palsy is another, but much rarer, complication. Both of these will typically resolve over time, but the healing process is slow. Patients with pre-existing nerve injury are at greater risk of experiencing this complication and are also slower to recover.[citation needed]

Chronic pain

A few patients who have had a hip replacement suffer chronic pain after the surgery. Groin pain can develop if the muscle that raises the hip (iliopsoas) rubs against the edge of the acetabular cup. Bursitis can develop at the trochanter where a surgical scar crosses the bone, or if the femoral component used pushes the leg out to the side too far. Also some patients can experience pain in cold or damp weather.[citation needed] Incision made in the front of the hip (anterior approach) can cut a nerve running down the thigh leading to numbness in the thigh and occasionally chronic pain at the point where the nerve was cut (a neuroma).

Death

The rate of perioperative mortality for elective hip replacements is significantly less than 1%.[44][45]

Metal-on-metal hip implant failure

By 2010, reports in the orthopaedic literature increasingly cited the problem of early failure of metal-on-metal prostheses in a small percentage of patients.

alloy
jewelry are more likely to have reactions to orthopedic implants. There is increasing awareness of the phenomenon of metal sensitivity, and many surgeons now take this into account when planning which implant is optimal for each patient.

On March 12, 2012, The Lancet published a study, based on data from the National Joint Registry of England and Wales, finding that metal-on-metal hip implants failed at much higher rates than other types of hip implants, and calling for a ban on all metal-on-metal hip prostheses.[52] The analysis of 402,051 hip replacements showed that 6.2% of metal-on-metal hip implants had failed within five years, compared to 1.7% of metal-on-plastic and 2.3% of ceramic-on-ceramic hip implants. Each 1 mm (0.039 in) increase in head size of metal-on-metal hip implants was associated with a 2% increase in failure rate.[53] Surgeons of the British Hip Society recommended that large head metal-on-metal implants should no longer be implanted.[54][55]

On February 10, 2011, the U.S. FDA issued an advisory on metal-on-metal hip implants, stating it was continuing to gather and review all available information about metal-on-metal hip systems.[56] On June 27–28, 2012, an advisory panel met to decide whether to impose new standards, taking into account findings of the study in The Lancet.[40][57][58] No new standards, such as routine checking of blood levels of metal ions, were set, but guidance was updated.[59] The U.S. FDA does not require hip implants to be tested in clinical trials before they can be sold in the U.S.[60] Instead, companies making new hip implants only need to prove that they are "substantially equivalent" to other hip implants already on the market. The exception is metal-on-metal implants, which were not tested in clinical trials, but, due to the high revision rate of metal-on-metal hips, the FDA has stated that, in the future, clinical trials will be required for approval, and that post-market studies will be required to keep metal-on-metal hip implants on the market.[61]

Modern process

Hip prosthesis 3D model
Different parts of hip prosthesis
A titanium hip prosthesis, with a ceramic head and polyethylene acetabular cup

The modern artificial joint owes much to the 1962 work of Sir John Charnley at Wrightington Hospital in the United Kingdom. His work in the field of tribology resulted in a design that almost completely replaced the other designs by the 1970s. Charnley's design consisted of three parts:

  1. stainless steel
    one-piece femoral stem and head
  2. Teflon
    ), acetabular component, both of which were fixed to the bone using
  3. PMMA (acrylic) bone cement

The replacement joint, which was known as the Low Friction Arthroplasty, was lubricated with synovial fluid. The small femoral head (78 in (22.2 mm)) was chosen for Charnley's belief that it would have lower friction against the acetabular component and thus wear out the acetabulum more slowly. Unfortunately, the smaller head dislocated more easily. Alternative designs with larger heads such as the Mueller prosthesis were proposed. Stability was improved, but acetabular wear and subsequent failure rates were increased with these designs. The Teflon acetabular components of Charnley's early designs failed within a year or two of implantation. This prompted a search for a more suitable material. A German salesman showed a polyethylene gear sample to Charnley's machinist, sparking the idea to use this material for the acetabular component. The UHMWPE acetabular component was introduced in 1962. Charnley's other major contribution was to use polymethylmethacrylate (PMMA) bone cement to attach the two components to the bone. For over two decades, the Charnley Low Friction Arthroplasty, and derivative designs were the most used systems in the world. It formed the basis for all modern hip implants. An example can be seen at the Science Museum, London.[62]

The Exeter hip stem was developed in the United Kingdom during the same time as the Charnley device. Its development occurred following a collaboration between Orthopaedic Surgeon Robin Ling and University of Exeter engineer Clive Lee and it was first implanted at the Princess Elizabeth Orthopaedic Hospital in Exeter in 1970.[63] The Exeter Hip is a cemented device, but with a slightly different stem geometry. Both designs have shown excellent long-term durability when properly placed and are still widely used in slightly modified versions.

Early implant designs had the potential to loosen from their attachment to the bones, typically becoming painful ten to twelve years after placement. In addition, erosion of the bone around the implant was seen on x-rays. Initially, surgeons believed this was caused by an abnormal reaction to the cement holding the implant in place. That belief prompted a search for an alternative method to attach the implants. The Austin Moore device had a small hole in the stem into which bone graft was placed before implanting the stem. It was hoped bone would then grow through the window over time and hold the stem in position. Success was unpredictable and the fixation not very robust. In the early 1980s, surgeons in the United States applied a coating of small beads to the Austin Moore device and implanted it without cement. The beads were constructed so that gaps between beads matched the size of the pores in native bone. Over time, bone cells from the patient would grow into these spaces and fix the stem in position. The stem was modified slightly to fit more tightly into the femoral canal, resulting in the Anatomic Medullary Locking (AML) stem design. With time, other forms of stem surface treatment and stem geometry have been developed and improved.

Initial hip designs were made of a one-piece femoral component and a one-piece acetabular component. Current designs have a femoral stem and separate head piece. Using an independent head allows the surgeon to adjust leg length (some heads seat more or less onto the stem) and to select from various materials from which the head is formed. A modern acetabulum component is also made up of two parts: a metal shell with a coating for bone attachment and a separate liner. First the shell is placed. Its position can be adjusted, unlike the original cemented cup design which are fixed in place once the cement sets. When proper positioning of the metal shell is obtained, the surgeon may select a liner made from various materials.

To combat loosening caused by polyethylene wear debris, hip manufacturers developed improved and novel materials for the acetabular liners. Ceramic heads mated with regular polyethylene liners or a ceramic liner were the first significant alternative. Metal liners to mate with a metal head were also developed. At the same time these designs were being developed, the problems that caused polyethylene wear were determined and manufacturing of this material improved. Highly crosslinked UHMWPE was introduced in the late 1990s. The most recent data comparing the various bearing surfaces has shown no clinically significant differences in their performance. Potential early problems with each material are discussed below. Performance data after 20 or 30 years may be needed to demonstrate significant differences in the devices. All newer materials allow use of larger diameter femoral heads. Use of larger heads significantly decreases the chance of the hip dislocating, which remains the greatest complication of the surgery.

When available implants are used, cemented stems tend to have a better longevity than uncemented stems. No significant difference is observed in the clinical performance of the various methods of surface treatment of uncemented devices. Uncemented stems are selected for patients with good quality bone that can resist the forces needed to drive the stem in tightly. Cemented devices are typically selected for patients with poor quality bone who are at risk of fracture during stem insertion. Cemented stems are less expensive due to lower manufacturing cost, but require good surgical technique to place them correctly. Uncemented stems can cause pain with activity in up to 20% of patients during the first year after placement as the bone adapts to the device. This is rarely seen with cemented stems.[citation needed]

Techniques

There are several incisions, defined by their relation to the gluteus medius. The approaches are posterior (Moore), lateral (Hardinge or Liverpool),[64] antero-lateral (Watson-Jones),[65] anterior (Smith-Petersen)[66] and greater trochanter osteotomy. There is no compelling evidence in the literature for any particular approach.

Posterior approach

The posterior (Moore or Southern) approach accesses the joint and capsule through the back, taking

abductors and thus minimizes the risk of abductor dysfunction post operatively. It has the advantage of becoming a more extensile approach if needed. Critics cite a higher dislocation rate, although repair of the capsule, piriformis and the short external rotators along with use of modern large diameter head balls reduces this risk. Limited evidence suggests that the posterior approach may cause less nerve damage.[67]

Lateral approach

The lateral approach is also commonly used for hip replacement. The approach requires elevation of the hip abductors (gluteus medius and gluteus minimus) to access the joint. The abductors may be lifted up by osteotomy of the greater trochanter and reapplying it afterwards using wires (as per Charnley),[citation needed] or may be divided at their tendinous portion, or through the functional tendon (as per Hardinge) and repaired using sutures. Although this approach has a lower dislocation risk than the posterior approach, critics note that occasionally the abductor muscles do not heal back on, leading to pain and weakness which is often very difficult to treat.

Antero-lateral approach

The anterolateral approach develops the interval between the

tensor fasciae latae
and the gluteus medius. The Gluteus medius, gluteus minimus and hip capsule are detached from the anterior (front) for the greater trochanter and femoral neck and then repaired with heavy suture after the replacement of the joint.

Anterior approach

The anterior approach uses an interval between the sartorius muscle and tensor fasciae latae. This approach, which was commonly used for pelvic fracture repair surgery, has been adapted for use when performing hip replacement. When used with older hip implant systems that had a small diameter head, dislocation rates were reduced compared to surgery performed through a posterior approach. With modern implant designs, dislocation rates are similar between the anterior and posterior approaches.[68] The anterior approach has been shown in studies to variably improve early functional recovery, with possible complications of femoral component loosening and early revision compared to other approaches.[35][33][69][70][71][72]

Minimally invasive approaches

The dual incision approach and other minimally invasive surgery seeks to reduce soft tissue damage through reducing the size of the incision. However, component positioning accuracy and visualization of the bone structures can be significantly impaired as the approaches get smaller. This can result in unintended fractures and soft tissue injury. The majority of current orthopedic surgeons use a "minimally invasive" approach compared to traditional approaches which were quite large comparatively.

Computer-assisted surgery and robotic surgery techniques are also available to guide the surgeon to provide enhanced component accuracy.[73] Several commercial CAS and robotic systems are available for use worldwide. Improved patient outcomes and reduced complications have not been demonstrated when these systems are used when compared to standard techniques.[74][75]

Implants

Metal on metal prosthetic hip
Cement-free implant sixteen days after surgery. Femoral component is cobalt chromium combined with titanium which induces bone growth into the implant. Ceramic head. Acetabular cup coated with bone growth-inducing material and held temporarily in place with a single screw.

The prosthetic implant used in hip replacement consists of three parts: the acetabular cup, the femoral component, and the articular interface. Options exist for different people and indications. The evidence for a number of newer devices is not very good, including: ceramic-on-ceramic bearings, modular femoral necks, and uncemented monoblock cups.[76] Correct selection of the prosthesis is important.

Acetabular cup

The acetabular cup is the component which is placed into the

Morse taper.[citation needed
]

Femoral component