SHORT NOTES ON JOINT REPLACEMENT SURGERY-BIOMECHANICS NOTES

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JOINT REPLACEMENT SURGERY

Joint replacement is one of the most common and successful operations in modern orthopaedic surgery. It consists of replacing painful, arthritic, worn or diseased parts of the joint with artificial surfaces shaped in such a way as to allow joint movement.

Arthroplasty is a common but loose term for joint replacement. Other types of surgery are also arthroplasties. Other common and valid synonyms are total joint replacement, total joint arthroplasty, joint resurfacing and artificial joint surgery.

Technique

Joint replacement is major surgery. The joint must be exposed and dislocated. The joint surface and some bone tissue is then removed from the bone ends and the prosthetic components implanted. They may be fixed by an interference fit with the expectation of bone-ingrowth or using PMMA “cement” as a grout to hold the metal components into the bone. The dislocation of the joint is reduced and the ligaments and muscles over the joint are repaired where possible.

Variations

There are many variations in the exact shape and design of the components and the technique and instruments needed to place them correctly. Although these design innovations are all driven by the impetus to improve results, most of the benefits are unproven. The results are already so good that very large, powerful studies are needed to demonstrate improvement from 95% success to anything better than that.

The main variations in technique are cemented vs cementless fixation; resurfacing or more radical removal of bone; and minimally invasive technique where the exposure is more limited.

Indications

Joint Replacement surgery is indicated when the symptoms, usually pain and loss of function, are disabling. As the risks of surgery are significant, the patient must understand them and prefer to take those risks rather than continue with the symptoms.

Contra-indications

Purulent discharge (infection) in the operative area is considered an absolute contra-indication because of the disastrous consequences of post-operative deep infection. Infection anywhere in the patient is a strong but relative contra-indication.

Poor health is a relative contra-indication as the patient must be strong enough to withstand the stresses of major surgery. Some feel that persistent immobility due to pain is a more serious threat to health even in patients with severe heart and lung disease.

Pre-operative work-up

Because of the major surgery a complete pre-anaesthetic work-up is required. In elderly patients this usually would include ECG, Chest Xray, urine tests, haematology and biochemistry blood tests. Cross match of blood is routine also as a high percentage of patients receive a blood transfusion. Pre-operative planning requires accurate Xrays of the affected joint. The implant design is selected and the size matched to the xray images (a process known as templating).

Post-operative rehabilitation

Early mobilisation of the patient is thought to be the key to reducing the chances of complications such as venous thromboembolism and Pneumonia. Modern practice is to mobilize patients as soon as possible and ambulate with walking aids when tolerated. Depending on the joint involved and the pre-op status of the patient the time of hospitalization varies from 1 day to 2 weeks with the average being 4-7 days in most regions.
Physiotherapy is used extensively to help patients recover function after joint replacement surgery. A graded exercise programme is needed. Initially the patients’ muscles have not healed after the surgery; exercises for range of motion of the joints and ambulation should not be strenuous. Later when the muscle is healed the aim of exercise expands to include strengthening and recovery of function.

Timecourse of recovery

A few days hospitalization followed by several weeks of protected function, healing and rehabilitation. This may then be followed by several months of slow improvement in strength and endurance.

Risks and complications:

Medical risks

The stress of the operation may result in medical problems of varying incidence and severity.

* Heart Attack
* Stroke
* Venous Thromboembolism
* Pneumonia
* Increased confusion
* Urinary Tract Infection (UTI)

Intra-operative risks

* Mal-position of the components
o Shortening
o Instability/dislocation
o Loss of range of motion
* Fracture of the adjacent bone
* Nerve damage
* Damage to blood vessels

Immediate risks

* Infection
o Superficial
o Deep
* Dislocation

Medium-term risks

* Dislocation
* Persistent pain
* Loss of range of motion
* Weakness
* Indolent infection

Long-term risks

* Loosening of the components: the bond between the bone and the components or the cement may breakdown or fatigue. As a result the component moves inside the bone causing pain. Fragments of wear debris may cause an inflammatory reaction with bone absorption which can cause loosening. This phenomenon is known as osteolysis.
* Wear of the bearing surfaces: polyethylene is thought to wear in weight bearing joints such as the hip at a rate of 0.3mm per year. This may be a problem in itself since the bearing surfaces are often less than 10 mm thick and may deform as they get thinner. It is also a problem because the wear debris may cause problems.

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