Knee Joint Replacements - What a GP Needs to Know

Knee replacement surgery has improved dramatically over the last few decades because of improved techniques, improved materials and improved understanding of knee physiology and function¹. More successful and durable knee replacement has had a big impact on clinical practice. GPs need to know something of the advances that have been made and also how clinical practice has changed as a result, particularly in the area of patient assessment and selection for surgery. GPs should be able to distinguish normal from abnormal progress after surgery.

Knee joint replacement has been performed for more than 50 years. Although it was attempted in the 1860s the first artificial implants were not tried until the 1940s. Problems with postoperative pain and loosening limited success. The success with hip arthroplasty was encouraging but the complexities of the knee joint hindered similar progress.The simple hinge like prostheses of the 1950s did not take into account the complexities of knee mechanics and high rates of failure with aseptic loosening resulted. Infection also contributed to an unacceptable failure rate. In the 1970s a joint which took into account the more complex movement of femoral condyles over the tibia was developed by Frank Gunston (a Canadian orthopaedic surgeon from Sir John Charnley's Hip Centre) but failed through inadequate fixation of the prosthesis. By 1974 replacing the patellofemoral joint (either preserving or sacrificing the cruciate ligaments) had become the standard operation.² More natural kinematics can be aided by retaining cruciate ligaments.

By 1994 Ranawat reported the best results so far with 95.6% success at 14 years follow up³ and excellent durability for total condylar knee replacement in1999 at 20 year follow up.⁴ In 2002 there were 19 different companies in the United States distributing knee implants used in about 130,000 knee replacements. These comprise 3 types (cruciate-preserving, cruciate substituting and TC-III). The success of knee replacement surgery has now equalled, and perhaps even surpassed, that of hip replacement.

There are broadly speaking 3 types of knee replacement:

• Unicompartmental knee replacement.
  • This involves replacement of medial or lateral tibiofemoral compartment only with components as below
  • It has largely replaced osteotomy
  • It is usually used for osteoarthritis
  • It is not suitable for patients with rheumatoid arthritis.
• Unconstrained bicompartmental knee replacement.
  • The most common form of total knee replacement
  • The femoral component is metal (and anatomically similar to the bone it replaces with a groove for the patella)
  • The tibial component is a flat metal platform with a plastic (polyethylene) cushion and a stem extending into the tibia
  • The patella component (button) is a dome shaped piece of polyethylene anchored to a flat metal plate.
• Constrained bicompartmental knee replacement.
  • These are used much less often when considerable bone loss is involved, for example with bone tumours
  • It is not possible to preserve the more physiological joint action with preservation of supporting ligaments etc and the joint works like a fixed hinge
  • It is much more prone to loosening.

There are over 150 knee replacement designs on the market. Choice of prosthesis depends on many factors (including age, weight, level of activity, health, cost of prosthesis and experience of the surgeon).
The aim of knee replacement is to replace the deficient joint surfaces (Condyles of the femur and top surface of the tibia) with low friction artificial surfaces (including occasionally the patellofemoral joint if affected). The best function and outcome is in general achieved by restoring mechanical alignment and soft tissue balance. This with newer designs means taking advantage of support from posterior cruciate ligament and collateral ligaments. Components are designed so that metal (titanium or cobalt/chromium based alloys) articulates with plastic (ultra high density polyethylene). A complete prosthesis now weighs only about 15 to 20 ounces.

• Osteoarthritis (OA) is the most common reason for knee joint replacement. This may be:
  • Primary osteoarthritis. Some 40% of 40 year old will have radiographic evidence of osteoarthritis of which half will have symptoms. There is a genetic risk for this with siblings of patients having joint replacement being up to 5 times more likely to need similar surgery.
  • Secondary osteoarthritis. This is secondary to mechanical derangement (for example meniscal or cruciate ligament injuries), infection, instability, fracture into the joint.
• Other causes of cartilage destruction:
  • Rheumatoid arthritis
  • Haemophilia
  • Seronegative arthritis
  • Avascular necrosis. This can be idiopathic or steroid-induced.
  • Gout and other crystal deposition diseases
  • Rare causes include bone dysplasias and bone cancers (e.g osteosarcoma). The bone destruction involved usually requires the use of more unusual 'fixed hinge' or constrained knee joint prostheses.

History

The basic indications are pain and loss of function. However the assessment of these is complex and it is difficult to define a clear consensus between surgeons.⁵ Selection for surgery should take into account severity of symptoms, the combination or integrated effect of these symptoms and the patient's motivation to proceed. Factors to be considered include:

• Age. Surgeons are less likely to operate on young patients.
• Pain. This is predominantly on weight bearing initially and then becomes more constant. It may be generalised or localised to one compartment (usually the medial compartment). Pain can be graded either from 1 to 10, or as either mild, moderate or severe. Severe daily pain with rest pain on several days per week and daily pain on transferring would help persuade surgeons to operate.
• Functional impairment. How are activities of daily living affected? Are walking devices or aids are used? What is the maximum walking distance? Can the patient climb stairs? Can the patient dress and look after themselves? What other activities requiring knee bending are impaired?
• Sleep. Sleep may be disrupted by pain. This can lead to depression and mood disturbance.
• Sexual and social functioning
• Comorbidity. Osteoarthritis and rheumatoid arthritis are perceived as having a good outcome from surgery.⁵
• Technical difficulties
• Patient motivation
• Employment and maintenance of independent living.

Scoring systems have been used in research in an attempt to quantify disability and measure outcomes. These include: Measures of general health status and specific knee scoring systems (eg via the Knee Society^6,7).

Examination

Examination should incorporate:

• General health and assessment of any medical conditions (for example hypertension, heart disease, diabetes etc).
• Knee examination
  • Inspection. Look for scars, skin sepsis and soft tissue defects around the knee. Skin defects or scarring may affect healing.
  • Palpation. Check pulses and ensure no peripheral vascular disease. Arrange for further investigation if peripheral vascular disease suspected.
  • Look for wasting of quadriceps and check good extensor function
  • Measure range of joint movement and any fixed flexion deformity (ideally using goniometer) and record findings. The best predictor of range of movement after operation is the range before operation.

Investigations

• X-ray (PA, AP, lateral, skyline view of patella, 45 degree PA or Rosenberg view). These should correlate with the clinical findings. Radiographic destruction of joint space (taken with symptoms) are required findings by most surgeons. Cysts, subchondral sclerosis and osteophytes are also seen in OA.⁵ The Knee Society have also endorsed a scoring system for X-rays after arthroplasty.⁸
• MRI scan. This may be done to further assess particularly cruciate ligaments and the patellofemoral compartment.
• Indium white cell scanning and other techniques are rarely needed before operation. In complicated revision arthroplasty where infection needs differentiating from aseptic loosening various techniques can be used.⁹
• Bone densitometry. This is indicated where there are risk factors for osteoporosis.
• Arthroscopy. This has often been performed on younger patients to evaluate symptoms, injury etc.

Measures to delay or avoid knee replacement

Conservative measures to be considered before surgery include:

• Adequate pain management. This should include oral drugs (non steroidal inflammatories etc) as well as other measures such as intra-articular viscosupplementation.
• Weight loss
• Physiotherapy to help with muscle strengthening, gait, patient education, joint mobility, walking aids
• Occupational therapy to aid with activities of daily living: bath aids, toilet aids, grab rails etc.
• Walking aids (shoe adjustments, bracing of joints etc).

• Knee sepsis
• Poor knee extensor function
• Significant vascular disease
• Recurvatum knee deformity.

Other conditions which cause significant difficulty include:

• Any condition increasing anaesthetic risk and good recovery significantly
• Skin disorders around the line of incision
• Past history of osteomyelitis
• Obesity
• Neurological disorders affecting musculature or joint.

It may useful for the GP to have details about the operation where they have a bearing on postoperative progress and possible complications. Such detail is seldom routinely provided but could include:

• Method of anaesthetic. Often epidurals are used.
• Tourniquet time
• Operative approach. This is usually anterior through a medial parapatellar approach (may be lateral or subvastus). Mini-incision techniques may be used. Guidance from NICE has been produced.¹⁰
• Whether good mechanical alignment achieved with good patellofemoral alignment
• Whether knee ligaments were significantly contracted and if so whether knee ligament release was necessary
• Whether patellofemoral surgery required (release, reefing or even resurfacing with polyethylene)
• Detail of the prosthesis and whether cemented or uncemented.

• In hospital, postoperatively, early knee movement, within the first 24 hours, is encouraged with:
  • Good analgesia. Often patient controlled methods. Epidural methods also commonly used.
  • Physiotherapy. Continuous passive motion machine may be used. Exercises taught. Most patients walk on the 2nd postoperative day.
  • Cryotherapy may be used to reduce swelling.
• Early discharge is encouraged at between 5 days and 2 weeks depending on progress, but only if:
  • Wound healing is satisfactory
  • Mobility is satisfactory
  • Knee flexion of 90 degrees is achieved
  • No complications have been identified.
• Orthopaedic follow up is usually at about 6 weeks in outpatients
• Review in general practice is likely to be for:
  • Analgesia
  • Wound care
  • Reassurance
  • Identification of complications
  • Review of thromboembolism prophylaxis. This is often continued at home.
  • Review for return to normal activities:¹¹
    • Driving:if left knee replaced and automatic, then driving can be resumed as soon as 1 week after surgery. If right knee replaced 4-6 weeks off driving is to be expected.
    • Return to work. This depends on type of work, but may take up to 8 weeks.
    • Travelling. Measures to prevent thromboembolic complications are recommended. Airport metal detectors may be triggered and medic alert card can be useful.
    • Sleeping positions. Sleeping on back, side and stomach is safe.
    • Other activities:swimming is recommended, usually from the time sutures are removed, and wound healed fully (about 6 weeks). Dancing, golf (with spikeless shoes and cart), cycling (level ground) are also acceptable. Activities which stress the joint should be avoided ( for example:tennis, squash, jumping, skiing, jogging).

• Thromboembolic complications including deep vein thrombosis and pulmonary embolus
• Infection
• Patellofemoral instability and other complications
• Vascular complications
• Neurological complications. Peroneal nerve palsy is the most common neurological problem occurring most often when correcting fixed valgus and flexion deformities. 50% recover spontaneously.
• Aseptic loosening
• Fractures. Periprosthetic fractures, especially of the femur (supracondylar) can occur rarely.
• Arthrofibrosis or restrictive scarring can impair knee movement.

New developments include:

• Use of uncemented designs allowing bone to grow into a porous prosthesis and give 'biologic' fixation
• New joint surfaces (eg bioactive surfaces like hydroxyapatite)
• Mobile bearing knee replacements are being improved. In a mobile-bearing knee replacement both the femoral and tibial components move across a polyethylene insert creating a dual surface articulation and helping to reduce wear and loosening.
• Improved kinematics
• Improved fixation
• Use of navigation guided surgery. Mini-incision for knee joint replacement is another term for use of navigation guided instruments which allow smaller incisions and less tissue destruction. NICE guidance from 2005¹⁰ recommends that more research is needed and that patients should be entered into suitable trials. The prostheses used are essentially the same.

If carefully and correctly selected for surgery a good outcome is expected. There should be relief of pain and restoration of function, even allowing patients to participate in some gentle sporting activity. Longer term studies are showing low failure rates of around 5% after 12-15 years. Cementless prostheses have not been followed up for as long.

Reducing the need for knee replacement will depend largely on progress in the prevention of osteoarthritis.