Clinical Review - Hip and knee replacements

Contributed by Mr Lee Jeys, consultant orthopaedic surgeon at the Royal Orthopaedic Hospital, Birmingham, and director of the Midland Hip and Knee Clinic

X-ray of a patient with superior pole arthritis limiting walking to 250m
X-ray of a patient with superior pole arthritis limiting walking to 250m

Section 1 Advice for patients
In 2007/8 approximately 150,000 hip and knee replacement operations were performed in the UK.1

Joint replacements are primarily pain-relieving operations and will only relieve pain emanating from the joint being replaced. Therefore accurate assessment of other sources of pain is mandatory.

Hip pathology normally causes groin, knee, anterior or lateral thigh pain.

Knee pathology is usually easier to discern, with pain most frequently being located to the joint line, anterior knee or shin.

Criteria for surgery
The proportion of patients aged under 60 at surgery has dramatically increased over the past 10 years (from 8 per cent to 23 per cent)2 and in most patients life expectancy is now greater than that of the implant.3

The mean BMI of patients undergoing joint replacement is also increasing. Obesity is associated with a slight increase in early complications, but no difference in functional scores or long-term implant survival.

Increased BMI is therefore not justification for refusing surgery.4 However, surgery is more difficult and studies have shown that patients do not lose weight following joint replacement.

Outcomes
Total joint arthroplasty has shown to significantly improve patient's quality of life and function long term.

Studies show that 85 per cent of patients would rate their outcome as good or excellent5 and the risk of complications is relatively low.

Outcomes have been shown to be significantly better in hospitals or with surgeons who perform a high volume of joint replacements.6

Recovery
Patients are normally required to use two walking sticks for six weeks after a hip or knee replacement, often performing a home exercise programme rather than formal physiotherapy.

Patients with cemented hip replacements are normally required to sleep on their back for 12 weeks following surgery and require toilet/chair raises for a similar length of time, to reduce the risk of dislocation.

In the longer term, patients should avoid crossing their legs, low seats and excessive hip flexion beyond 90 degrees.

Patients with larger bearing surfaces (>36mm) often have no restrictions postoperatively and are allowed to return to sports.

Patients with knee replacements often report initial pain and take a few days to be able to straight leg raise, due to quadriceps inhibition. Working on knee flexion and extension is important in the first six weeks.

Knee replacements should correct any pre-operative deformity but postoperative range of motion is related to pre-operative range of motion.

Kneeling may be painful after a knee replacement, whether the patella has been resurfaced or not.

Section 2 Types of hip and knee replacements

Hip resurfacing
In hip resurfacing, the hip joint is surgically dislocated and the femoral head is shaped to accommodate a thin, cemented metal cap with a short stem that does not breach the femoral shaft canal. This cap articulates with a metal uncemented cup.

There are several theoretical advantages to the replacement, including reduced wear rates, the ability for the head to be nearly anatomic in size, thus reducing the risk of dislocation, and the fact that it preserves the femoral neck, making leg length discrepancy unlikely and preserves bone stock for revision surgery.

However, there is a risk of femoral neck fracture, particularly in perimenopausal women.

The technique is a much more invasive exposure than a routine hip replacement and there are some cases of subsequent avascular necrosis to the femoral head. It is therefore best suited to younger males with osteo-arthritis.

Cemented hip replacements
Cemented hip replacements remain the gold standard. They comprise a metallic femoral component, held inside the femoral canal by bone cement containing antibiotics, paired with a cemented ultra-high molecular weight polyethylene (UHMWPE) cup.

The main disadvantage is that the femoral head has to be small, leading to significant restrictions on the patient's lifestyle.

Cemented hip replacements remain the most common prostheses inserted in the UK, and the long-term survival results remain the best currently.

Uncemented hip replacements
Uncemented hip replacements now make up 40 per cent of all replacements. Modern uncemented replacements are coated with hydroxyapatite, allowing bone to grow into or onto the implant, bonding it to the bone.

Modern uncemented hip replacements have long-term results comparable with cemented hip replacements, and are often used for younger or more active patients.

They allow the use of large heads or new bearings (such as ceramic on ceramic) and do not have the problems with hip fracture, therefore offer some advantages over hip resurfacing.

The disadvantages are that they are more expensive than their cemented counterparts, are more difficult to revise and if they have problems with initial fixation, can produce thigh pain or have small fractures on implantation.

Knee replacements
Most knee replacements in the UK are bicondylar, which resurface the femoral joint surface with a metal cap, the tibial joint surface with a metal and UHMWPE liner and some surgeons resurface the patella with a UHMWPE button.

Established knee replacement designs have 95 per cent 10-year and 85 per cent 20-year implant survival results.

Unicompartmental or 'half' knee replacements are used if there is significant arthritis in the medial or lateral compartment, minimal deformity, a functional anterior cruciate ligament, and in young patients. They replace the tibial surface and the posterior condyle of the femur on the affected side.

The long-term results are less good than a total knee replacement, but they can usually be easily revised to a standard total knee replacement.

Section 3 Complications
Hip replacements

The majority of hip replacements are pain-free after the operation and continuing pain should be viewed with suspicion.

Ongoing pain can be associated with trochanteric bursitis or abductor fatigue. It is common between four and 12 weeks after the hip replacement for patients to complain of buttock pain on walking over half a mile and this is usually due to abductor fatigue, which will ease with exercise or physiotherapy.

Other common problems include a Trendelenburg gait (wobbly gait due to abductor weakness), or dependent leg oedema, which will resolve spontaneously but may take several months.

Patients with dependent oedema normally notice that the leg swelling increases throughout the day, but improves with overnight support stockings and sleeping with the bed elevated.

Infection
One of the major complications is infection, and superficial wound infections are common, with 5 per cent requiring a course of antibiotics. If the wound remains infected there is a 20 per cent chance of deep periprosthetic infection. The infection forms a biofilm around the metallic implants making eradication very difficult.

Treatment of a deep infection includes washing the hip joint out in the early course, but if this is unsuccessful, then revision surgery is required.

Dislocation of the hip replacement is a common complication, with reported rates being 0.3-5 per cent. The risk of dislocation is highest in the first three months following surgery. If a dislocation occurs, it is associated with severe pain, the leg is often short and internally rotated.

Vascular complications
Thromboembolism is a major risk with patients undergoing hip replacement. NICE guidance states that patients over the age of 60 or with another risk factor should have extended prophylaxis with a low molecular weight heparin (LMWH) for 28 days following surgery, although this is controversial due to the increased risks of bleeding.

The estimated risks of DVT and fatal PE with prophylaxis are 5 per cent and 0.5 per cent respectively.

Lesser complications of hip replacements include leg length discrepancy, which can cause a slightly poorer functional outcome and more pronounced limp.

A rarer complication is a nerve palsy, most commonly the sciatic nerve, which occurs in less than 1 per cent of cases.

Knee replacements
The systemic complications of knee replacements are similar to hip replacements, with a similar rate of infection, thromboembolism and medical complications. NICE guidelines suggest that prophylaxis with LMWH should be continued for 14 days following surgery.

The most common three problems following surgery are pain, stiffness and instability.

At one year from surgery 80 per cent of patients are satisfied and persistent pain was the most common reason for dissatisfaction. This group of patients need careful investigation to rule out infection, instability, patello-femoral pain or loosening.

However, if no treatable cause can be found a conservative approach is more likely to offer pain relief than revision surgery.

Limited movement
Postoperative stiffness is common following knee replacement and a poor range of motion is often associated with levels of pre-operative pain, movement and anxiety and the inability to perform adequate rehabilitation. Manipulation under anaesthesia within six months from surgery can help to improve the range of motion and decrease pain levels.

Pre-operative deformity or incorrect ligament balancing can result in the knee replacement becoming unstable.

Excessive strains on the implant and soft tissue can cause pain, or the knee gives way under the patient causing unsteadiness or falls. Physiotherapy and revision surgery to a more constrained joint replacement are often required.

Section 4 Revision surgery
Knee and hip revision surgery has become a separate subspecialty, and 10 per cent of all joint replacement operations within the UK in 2007/8 were revision operations. It is estimated that numbers will double in the next 10 years.

Specialist techniques
Specialist techniques are required to remove a well-fixed implant or the cement mantle (leaving the cortex intact), eradicating infection and dealing with bone loss.

New technologies such as ultrasonic cement removal, special implants (long-stemmed or constrained implants), silver coated implants (acts as antimicrobial), or porous metallic augments (allowing bone ingrowth) may be required to deal with these issues, which make operations expensive with prolonged hospital stays and increased risks of complications.

However, excellent functional results are achieved in more than 70 per cent of cases and long-term implant survival is possible (70 per cent at 15 years), although the proportion of patients achieving both is less than for primary surgery.

References

1. 5th National Joint Registry report (www.njrcentre.org.uk)

2. Chidambaram R, Cobb A. Change in the age distribution of patients undergoing primary hip and knee replacements over 13 years - an increase in the number of younger men having hip surgery. J Bone Joint Surg (Br) 2009; 91-B: Supp_I, 152.

3. Ramiah R D, Ashmore, A M. Whitley E, Bannister G C. Ten-year life expectancy after primary total hip replacement. J Bone Joint Surg (Br) 2007; 89-B: 1,299-302.

4. Amin A, Sales J, Brenkel I. Obesity and total knee and hip replacement. Curr Orthop 2006; 20(3): 216-21.

5. Espehaug B, Havelin L, Engesaeter L et al. Patient satisfaction and function after primary and revision total hip replacement. Clin Orthop Relat Res 1998; 351: 135-48.

6. Doro CJ. Hospital volume and outcomes of total hip arthroplasty in the United States. J Arthroplasty 2006; 21(2): 304.

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