Boundary lubrication depends on molecules sliding over each other and is best for heavy loads and slow movements. Graphite is a boundary lubricant. The large hyaluronic acid-based proteoglycan molecules in synovial fluid function in the same way.

Compounds which flow between surfaces, such as oils, are the best known lubricants. Synovial fluid acts as a surface lubricant and its efficiency is increased by the ‘wedge effect’. The joint surfaces are not a perfect fit and the irregularities create a wedge of fluid at the point of contact, a little like a car ‘hydroplaning’ on a wet road.

To improve lubrication still further, fluid from the articular cartilage is squeezed out when it is compressed. This is ‘boosted’ lubrication.

Stainless steel, perhaps the simplest implant material, contains a cocktail of different elements and is generally satisfactory, although not as strong as chrome, cobalt and molybdenum alloys of which many implants are made (Table 4.1). Prostheses are also made of almost pure titanium. Precious metals such as silver and gold are satisfactory implant materials but lack the strength required of a prosthesis, quite apart from their cost.

Most artificial joints consist of a metal and plastic articulation. The plastic usually used is high density polyethylene (HDP), but some early prostheses were made of polytetrafluoroethylene (PTFE, Teflon), which was satisfactory until its wear particles provoked a vigorous foreign body reaction that eroded bone. Particles of HDP also produce an inflammatory response, but it is much milder than that caused by PTFE.

Ceramics are also used in prostheses but they are brittle and their wear particles are irritant.

An interference fit is nothing more than a tight fit like a cork in a bottle or a nail in wood. Some implants have fins and flanges to make them fit more firmly at operation, but sound immediate fixation does not prevent the implant loosening with time.

Screws produce sound fixation of a plate to bone but are themselves only held by an interference fit and can loosen.

Bone cements are not adhesives; they do not ‘stick’ to bone but fill up spaces to produce a better mechanical fit. In builders’ language, bone cement is a grout, like the material used to fill the cracks between tiles.

The most commonly used is acrylic cement, which is polymethylmethacrylate and chemically the same as Perspex. The cement is prepared during operation by mixing a liquid which contains the monomer (monomethylmethacrylate) and a stabilizer to prevent it polymerizing, with a powder that includes a catalyst to initiate polymerization, a filler consisting of polymethylmethacrylate powder, a radiopaque material such as barium sulphate, and sometimes an antibiotic. The mixture forms a dough-like material which can be forced into the medullary cavity around the implant, where it sets solid. Low viscosity cement with the consistency of thick cream is also available. Although more difficult to handle, low viscosity cement can be forced into cancellous bone, hopefully to achieve more secure fixation.

Acrylic cement is the best available at present but has many disadvantages. While strong in compression, it is weak in tension and breaks when twisted. Once fractured, the two surfaces rub together and produce wear particles which are irritant to bone. A brisk foreign body reaction results, more bone is resorbed, more acrylic cement loosens, more particles are formed and a vicious circle is established, leading to irreversible loosening of the implant and bone destruction.

One way of overcoming the problem of bone fixation is to coat the prosthesis with a porous surface of sintered metal granules, so that bone may grow into the minute passages that cover the prosthesis and thus fix it to the bone. However, even if the bone does grow into the prosthesis, loosening and fracture may still occur unless the prosthesis has the same elastic properties as the bone. A coating of hydroxyapatite to the component can also be used. This again allows the bone to grow up to the edge of the component and integrate with the coating. Ligament prostheses can be fixed to bone by making a bone/prosthesis/bone sandwich so that the cancellous bone grows through the weave of the prosthetic material. Fixation of fabric prostheses occurs readily in soft tissues, both in arterial grafts and hernia repairs.

The timing is very variable and is much faster in children, in whom callus can be seen at 2 weeks.

In children, remodelling will correct some, but not all, bone deformities. Up to 30° of malalignment in the plane of movement of the joint can be expected to correct itself by remodelling in the young child. Rotational deformities and malalignments in other planes do not do so well (Fig. 4.8 and Fig. 4.9). A further blessing is that, in children, a broken limb will grow faster than normal and a loss of length of 1–1.5  cm can be accepted under the age of 12 years.

Hyaline cartilage does not regenerate in the adult. Although superficial damage can heal in the very young, for all practical purposes injuries to hyaline cartilage heal with fibrocartilage and fibrous tissue with inferior weight-bearing properties (Fig. 4.11).

Unlike bone, skin cannot reproduce itself and heals with a fibrous scar. This can be a special problem to the orthopaedic surgeon because any scar, even a surgical scar, that crosses a joint can contract and restrict movement (Fig. 4.12).

To operate upon a joint and create a scar which limits joint movement is not helpful and the site of incisions around joints must be carefully selected. As a general rule, incisions should never cross skin creases on the flexor surface of joints.

Nerves are often severed in limb trauma and small cutaneous nerves may be divided during operation. When a nerve is cut, changes are seen in the cell body and the axon cylinder distal to the cut degenerates.

Fine fibrils from the proximal end enter the distal sheath and grow down it at the rate of 1 mm per day. Healing of the nerves depends upon the two cut ends of the nerve being so close that the axon can cross from the proximal end of the nerve to the distal and grow down its own axonal tube to the end plate. Although individual nerve fibres may heal well and nerves can be approximated surgically using a microscope so that they appear anatomically normal, there is no guarantee that each individual neurone will find its correct destination. A telephone cable provides a useful analogy: to cut across a large telephone cable and hold the two ends together with tape is unlikely to make all the right connections (Fig. 4.13).

Because the nerve ends cannot be accurately apposed even with the help of a microscope, nerves do sometimes grow down the wrong axonal tube and reach the wrong end organ. The result of this can be that heat or light touch are experienced as pain and the incorrectly innervated skin is hypersensitive. If the cut ends are not approximated, a neuroma will form at the end of the nerve and this also leads to altered sensibility, which can be distressing.

Muscle, like skin, heals with fibrous tissue and a cut muscle never regains its full bulk or power, even if it does all that is asked of it in normal use. A few multinucleate muscle cells may be seen. These contribute little to the function of the injured muscle. Areas of ischaemic muscle following arterial damage, compartment syndromes or crushing injuries are replaced with a mass of fibrous tissue that contracts and limits joint movement (Fig. 4.14). In severe cases, the contracture will pull the limb down into a position of extreme flexion, as in Volkmann’s ischaemic contracture of the forearm.