Fixing a hip joint replacement to the skeleton
It is common to use the words ‘fixed’ and ‘loose’ when thinking about the fixation of metal and plastic parts to the bones.
However, neither of these two extremes can actually occur in joint replacement.
No component can be fully and solidly fixed without any movement whatsoever, even if it is not clear how much or where the movement occurs. The movement which takes place between the implant and the bone is governed by the behaviour of the tissues and materials in-between.
When a hip replacement is used in walking it is subject to cyclical loading, a regular mechanical stressing as each weight bearing step is taken. The behaviour of the interface between the implant and the body has a vital role in successful hip replacement.
The Five Facts of Implant Fixation
Just after insertion the implant is in contact with varying tissues - dead but intact bone, pieces of shattered bone, marrow, tissue debris, blood and blood clots. Living bone is unlikely to be in contact with the implant immediately after operation.
The patient’s natural reaction is inflammation in an attempt to heal the damage which occurred when the surgeon prepared the joint and inserted the implant. When the initial reaction of the patient to the joint replacement has been completed, a wide variation in tissue types may exist between the implant and the bone.
The closest contact between implants and the skeleton occurs when osseointegration has occurred, where living bone is separated from the implant material by a very thin biological layer. In cases of less close contact, layers of fibrous tissue or cartilage-like material may intervene between the implant and the body.
The area of initial bone death
The person has to deal with the area of bone death which surrounds the new joint replacement. The dead bone may be replaced by new living bone or absorbed and replaced by non-bony tissues.
Dead bone is weaker than living bone and more likely to fracture under stress. When the person gets up initially after operation the loads applied to the joint replacement are transmitted to the living bone through a layer of dead bone. This layer can be liable to failure which can lead to loosening. A large area of dead bone around the implant can make it impossible to create the osseointegration between the implant and the bone.
The hosts’ rate of bone turnover might be expected to be important here as they have to deal with replacing the area of dead bone with new living bone. However, there is no good evidence that this occurs, although some hips may loosen more in people who have lower than normal bone turnover. Some drugs affect bone formation and remodelling, so it would be surprising if they did not have an affect on the reaction to an implant.
The Stresses and Strains between the Implant and the Host
The development of the interface tissues between the joint replacement and the person depends on the stresses and strains applied to the area. The strength of the interlock achieved by the surgeon at operation determines these initial characteristics.
When the strength of the interlock is high and the loads applied to the joint are low then the stresses too are low. If the interlock strength is low and the loads applied are high then the stresses through the tissues will be high. This affects the development of the tissues which exist between the implant and the person.
A series of studies with metal fixation applied to bone illustrates the point. Where the fixation was stable the cells between the metal and the surrounding dead bone became bone producers and made normal bone. Where the fixation was more unstable and the stresses greater the cells changed into a type which formed fibrous tissue. Where the initial state of the fixation was stable and then was altered later to a less stable state the new bone being formed was absorbed again and fibrous tissue formed instead.
The host responds to the trauma of the operation by removing the dead and traumatized tissues and replaced them with living tissue. The type of new living tissue laid down depends on the stresses and strains applied to the tissue.
Where the stresses on the interface tissues are low and the movement at the interface is low then the result is bone formation. Where the interface stresses are high and the interface movement higher, the result is absorption of bone and formation of fibrous tissue or in some cases cartilage. These effects are reversible if the environment of the tissues change.
The most interesting area in current orthopaedic surgery is the position where the mechanical interlock created at operation is reasonably good and the stresses on the interface tissues are moderate and cyclical (ie repetitive).
Osseointegration may only be reliably expected when the trauma of the implantation is kept to a minimum, a strong interlock has been created and direct loading of the implant is postponed for three months. This allows the damaged area around the implant to be replaced by new living bone.
If loading is allowed too soon, fibrous tissue may be formed and osseointegration not achieved. Loosening is much more likely to occur in the latter group where the stresses have been too great. The acrylic cement used between the implants and the bone limits the strains applied to the interface tissues and may allow osseointegration to occur more predictably, even though stresses are being applied. In this case the cement acts not only to transmit load but also as a damper, reducing the loads applied across the interfaces.
Displacement of the interface under cyclical loads
The displacement which takes place between the implant and the bone depends on the properties of the interface tissues. Fibrous tissue developed at the interface has been shown to resist compression but not shearing stresses. As this layer may be up to 1.5mm thick it may have a significant effect on the behaviour of the joint.
Where osseointegration is present between the implant and the bone there is only a minute layer of biological material present. This restricts the allowed movement at the interface and allows direct transmission of the load when weight is applied.
It is not clear what amount of displacement is allowable to ensure painless normal function of an artificial joint. Swanson has said that the cyclical displacement of the artificial components is acceptable if it does not increase in size with regular loading, does not cause pain or significant wear of the implant. It should also not damage bone or cause it to be absorbed.
If there is a significant soft tissue layer interface between the implant and the bone, the only forces which can effectively transmitted are compression. Shear, rotational and tensile stresses are not transmitted well, and for this to be done there needs to be a large enough area of osseointegration or micro interlock.
Significance of osseointegration
Work has shown that loosening of components only occurred when loading was allowed before osseointegration developed. Then fibrous tissue was found between implant and bone. Once osseointegration has taken place loosening did not occur. The loading which is very damaging to the implant in the early stages is harmless once the interface is mature.
If adequate microinterlock and osseointegration are not established in the first few months of the implant’s life they are unlikely to occur later. This means a soft tissue interface is likely to develop. The first few months are critical.
The development of osseointegration may prevent the later migration of implant components which can occur without symptoms in both cemented and uncemented devices.
Relationship between osseointegration and microinterlock
Microinterlock was initially used to describe the situation when bone cement is forced under pressure into the bone spaces. If this is done well over a large enough area then it sets the initial necessary conditions which, along with avoiding excessive loads, allow the development of osseointegration.
Later events in the life of the interface
In the interface a vicious circle of bone destruction can be generated. Bone absorption may occur for a variety of reasons, but this increases the loads generated across the interface tissues. This can provoke further bone absorption, which itself means becomes easier to generate abnormal stresses in the interface. As this proceeds it may lead to gross loosening of the implant. This may lead to a revision operation becoming necessary.
Ling RSM, Observations on the fixation of Implants to the Bony Skeleton, Clinical Orthopaedics and Related Research, 210, 80-95, 1986.