1. OSTEOARTHRITIS

Osteoarthritis is a chronic degenerative knee disorder thought to be a consequence of evolution including a change in posture from quadruped to biped gait and the development of a long post-reproductive life span. Damage in the knee that appears under-designed for upright posture accumulates with age and no appropriate repair mechanism are in place to halt this process.

Despite the inflammatory connotation of the suffix 'itis', the initiating events in osteoarthritis appear to be mechanical rather than inflammatory in nature. Hence the terminology of osteoarthrosis and -arthritis are used equivocally. We distinguish between primary and secondary osteoarthritis. In most patients a definitive underlying pathology cannot be established, and these cases are considered to suffer from idiopathic or primary osteoarthritis. The true aetiology of primary osteoarthritis however is still unknown. Scientists have proposed the influence of nutritional and environmental factors but causation is difficult to evaluate. It has been known for a long time that osteoarthritis can present a strong family history or pattern of inheritance. It is therefore not surprising that there is already some evidence that genetic factors are at least partly responsible for the expression of arthritis (Jimenez & Dharmavaram 'Genetic aspects of familial osteoarthritis' Ann Rheum Dis 1994;53:789-797).

Secondary osteoarthritis is the result of various causes which adversely affect the structural integrity and biomechanical properties of cartilage, or the mechanical forces transmitted through the joint. These are typically associated with either systemic (e.g. rheumatism), metabolic (e.g. gout), and genetic disorders (e.g. achondroplasia), but can also be due to biomechanical abnormalities like varus/valgus mal-alignment, and bone/soft tissue injuries. Osteoarthritis is a continuous process of increasing damage to the joint surfaces. It usually starts by affecting the cartilage and menisci, which in time will wear away. Eventually the bone underlying the cartilage will deform and the joint will change in shape. Joint movements will be more and more restricted the further the disease process is progressing, and might terminate in a completely stiff and sometimes even pain free knee.

In the early stages when the damage might still be confined to a particular joint area or compartment it is sometimes possible to 'rescue' the joint by changing its alignment. If the damage is more severe a so-called partial or unicompartmental knee replacement can be implanted. In cases of advanced osteoarthritis however either a total knee replacement, or under certain circumstances (infection, very young patient) a knee fusion might be indicated. Arthroscopic procedures for arthritis are somewhat controversial (Garrett 'Evaluation and treatment of the arthritic knee' J Bone Joint Surg 85-A;156-157). Some scientists have even been suggesting that the placebo effect may be responsible for any benefit experienced by patients undergoing arthroscopic debridement for more advanced arthritic changes (Moseley et al. 'Arthroscopic treatment of osteoarthritis of the knee' Am J Sports Med 1996;24:28-34).

2. INFLAMMATORY ARTHROPATHIES

Inflammatory arthropathies representing a different disease group compared to osteoarthritis. However in both cases the accelerated wear and tear of the joint remains the main physical sign, causing significant disability to patients. Inflammatory arthropathies include rheumatoid arthritis as their main proponent. Others are psoriatic arthropathy, lupus, Morbus Reiter etc. All patients affected by this group of arthritis have in common that the disease process is not gradual but characterised by acute phases of deterioration and remission. Generally, however, joint degeneration is more rapid making joint replacement surgery often necessary at a much earlier stage in the disease process.

3. TREATMENT OPTIONS

3.1 PERIARTICULAR OSTEOTOMIES (straightening of bowed legs or knocked knees)

A high tibial or distal femoral osteotomy might be indicated if osteoarthritic changes are limited to one joint compartment, with all other compartments being substantially normal (Coventry 'Osteotomy about the knee for degenerative and rheumatoid arthritis' J Bone Joint Surg 1973;55-A:23-48). In order to judge upon the suitability and correct indication for a realignment procedure, arthroscopic assessment is sometimes required. The ideal patient for this procedure is below the age of 60, with no substantial limitation in knee range of motion and localised arthritic changes in one joint compartment only.

The high tibial closing wedge osteotomy was quite a commonly performed procedure, but lost some of its popularity due to reported problems with the applied technique. All closing osteotomies effectively shorten the leg, and create a lateral step on the tibia making future total joint arthroplasty more difficult. It also increases the patella height and henceforth may initiate patella subluxation and instability, especially in patients with known patello-femoral symptoms. Loss of correction is also a common problem and due to bone remodelling over time. It is therefore required to slightly over-correct the deformity, but most surgeons are often scared to remove a large enough bone wedge. Most of these problems can be overcome if instead of a wedge excision an opening wedge osteotomy is performed. The created gap is filled with bone graft and held open with a specifically designed plate that incorporates a metal spacer and is secured to the bone with screws (e.g. Puddu plate). The patient is mobilised without a plaster but requires crutches for about four to six weeks.

Correction of varus deformities can also be achieved by means of external fixation and bone transport. This technique allows for gradual correction until the mechanical axis is in the desired position. Adjustments are possible up to three weeks post surgery (from Dieter Kohn 'Knie' in Wirth & Zichner (eds.) 'Orthopädie und Orthopädische Chirurgie' Thieme 2005).

Most patients will present with arthritic changes affecting the medial joint compartment and which has created a so called varus or bowed leg deformity. The opening wedge osteotomy will change the leg alignment back into valgus. In cases of primary valgus or knocked knee deformity an opening wedge osteotomy is performed on the lateral aspect of the distal femur.

The alignment of the contra-lateral knee is usually used as a reference and correction varies between 5 to 20 degrees. The surgical aim is to change the mechanical leg axis by redistributing the weight away from the damaged, towards the un-damaged compartment. The damaged compartment is consequently offloaded, which in principle reduces the progression of surface wear and buys time before joint replacement surgery might be required. One has to be careful however not to overcorrect the alignment too much, as this might lead to increased pressure and subsequent osteoarthritis in the otherwise normal compartment. Unfortunately patients have to endure a prolonged recovery period of up to 6 months and a large proportion of patients will not be completely symptom free (Insall et al. 'High tibial osteotomy for varus gonarthrosis' J Bone Joint Surg 66-A;1040-1048).

3.2 UNICOMPARTMENTAL KNEE REPLACEMENT

The indications for performing a unicompartmental knee replacement are similar to the ones already outlined for high tibial osteotomies. A unicompartmental knee replacement is a surface replacement of articulating areas of femur and tibia (Scott 'Unicompartmental Total Knee Arthroplasty' In: Insall & Scott eds. 'Surgery of the knee' Churchill Livingstone, New York 2002:1621-1628).

Preferred materials are Cobalt Chrome and Polyethylene. The components can be attached to the bone with bone cement (Methylmethacrylate) or used uncemented. In uncemented replacements the undersurface of the implant is either Hydroxyapatite coated or has a porous surface, both of which will promote bone in-growth. Cemented replacements have the advantage of allowing patients full mobility and weight bearing from day one, whereas uncemented implants require a short period of protected weight bearing. Functionally unicompartmental knee replacements provide the patient with a mere physiological range of movement and hence are far less debilitating than total knee replacements (Goodfellow & O'Connor 'Clinical results of the Oxford knee' Clin Orthop 1986;205:21ff.).

The implantation of a unicompartmental knee replacement is technically quite demanding, as component alignment is crucial for long-term success (Sculco 'The case for total knee replacement in unicompartmental knee arthritis' Orthopedics 1994;9:857-858). Early failures are mostly related to over-correction or malpositioning of components. So far clinical results show that unicompartmental knee replacements are best suited for the replacement of the medial compartment but its successful application for localised lateral compartment arthritis has been reported on (Ashraf et al. 'Lateral unicompartmental knee replacement' J Bone Joint Surg 2002;84-B:1126-1130).

3.3 TOTAL KNEE REPLACEMENT

The first reliable total knee replacements were introduced in the early eighties superseding an array of earlier designs, which were hampered with a high failure rate (Insall & Clarke 'Historic development, classification, and characteristics of knee prostheses' In: Insall & Scott eds. 'Surgery of the knee' Churchill Livingstone, New York 2002:1516-1552). Like in unicompartmental replacements the distal femur and proximal tibia are replaced with metal components mostly made of cobalt chrome. In order to reduce friction a polyethylene insert is placed inbetween the femoral and tibial components. Today's knee replacements follow two different design principles. In fixed bearing replacements the polyethylene insert is rigidly attached to the tibial component, whilst mobile bearing replacements allow the insert to move somewhat independently between femur and tibia. The idea behind mobile bearings is based on the notion that a higher degree of freedom not only leads to a reduction in stress on component fixation but also increases the congruity between femoral component and polyethylene insert, hence reducing polyethylene wear (Insall 'Adventures in mobile-bearing knee design: a mid-life crisis' Orthop 1998;21:1021-1023, McEwen et al. 'Wear of fixed bearing and rotating platform mobile bearing knees subjected to high internal and external tibial rotation kinematics' J Mats Sci Mats In Med 2001;12:1049-1052).

Mobile bearing knee replacements are gaining more and more in popularity since long term follow up results have confirmed that they are at least as good if not superior to their fixed bearing counterparts (Buechel et al 'Twenty year evaluation of meniscal bearing and rotating platform knee replacements' Clin Orthop 2001;388:41-50, Pavone et al. 'Total condylar arthroplasty: a long term follow-up' Clin Orthop 2002;388:18-25, Price et al. 'A mobile-bearing total knee compared with a fixed bearing-bearing prosthesis' J Bone Joint Surg 2003;85-B:62-67). Both cemented and uncemented versions are available and bear similar advantages and disadvantages as mentioned with unicompartmental knee replacements. Clinical outcome studies of contemporary total knee replacement designs show survival rates of up to 97% at 10 years, which is equivalent to the survival of modern total hip replacements. The ideal patient for a total knee replacement is above 60 years of age and suffers from degenerative changes in at least two of the three knee compartments. Due to the geometry of total knee replacements it is usually not possible to give the patient more than 100º to 120º of knee flexion. However such movement is generally sufficient for most daily and recreational activities, with patients continuing to enjoy cycling, swimming, tennis and golf. Impact and cutting sporting activities including football and squash as well as high energy activities like some athletics should be refrained from in order not to jeopardise the longevity of the joint replacement as loosening is more likely to occur. Hence young and active patients are to be advised and properly counselled prior to surgery.

3.4 PATELLO-FEMORAL REPLACEMENT

In very few patients only the front part of the knee joint which is also called the patello-femoral joint appears to be affected by osteoarthritis (Merchant 'Patellofemoral joint disorders' In: Chapman ed. 'Chapman's Orthopaedic Surgery' Lippincott, Philadelphia 2001:2321-2337). These patients classically complain of disabling pain on ascending or descending stairs, squatting, kneeling and stiffness after prolonged periods of rest. In severe cases removal of the whole patella known as patellectomy was only too often seen as the treatment of choice. However today's technology allows us to selectively resurface the patello-femoral joint by placing a polyethylene button onto the retro-patella area and a metal component into the articulating trochlea groove of the femur (Argenson et al. 'Is there a place for patello-femoral arthroplasty' Clin Orthop 1995;321:162-167).

The long term outcome of patello-femoral replacements is however yet unclear but recent reports are promising (Krajca-Radcliffe & Coker 'Patellofemoral arthroplasty: a two to eighteen year follow-up study' Clin Orthop 1996;330:143-151). Problems with accelerated wear and patella dislocation in earlier designs have been addressed, but further follow up is needed to judge upon its validity.