A Method and Apparatus for Bone Graft Compaction

Background to the Invention

The present invention relates to a method and apparatus for bone graft compaction. It is particularly to be applied in methods of impaction bone grafting, for example in joint replacement surgery, particularly revision joint replacement surgery.

Revision joint replacement surgery (surgery to replace failed joint replacement prostheses, usually with new prostheses) accounted for approximately 15% of all joint replacement surgery carried out in the UK in 2005. This equates to approximately 7,500 revision hip replacements and a similar number of revision knee replacements per annum. The aging population and the trend for performing joint replacement operations in younger patients mean that the number of revision operations is likely to increase for the foreseeable future. Revision operations are costly and often their outcome is less successful than the primary operation.

In revision joint replacement surgery, a surgeon is often confronted by massive bone loss at the site of the existing prosthesis as a result of the loosening of the existing prosthesis. Impaction bone grafting is a technique for replacing this lost bone (described in patents WO0108569, US5470336, WO9740785 and WO9301773). By using the impaction bone grafting technique it is possible to simultaneously restore the anatomy of the patient to a more normal state and to provide a solid foundation for the new prosthesis. The restoration of anatomy differentiates impaction bone grafting from other techniques such as bulk cementation or use of oversized or extra-long prostheses to bypass the defective site. The solid foundation of bone produced by the impaction bone grafting technique enables the new prosthesis to function properly, giving pain relief and improved mobility for the patient, and, unlike alternative techniques, the probability of success of any subsequent revisions to be improved.

In the impaction bone grafting procedure, small chips of bone, which may be obtained from a donor, and/or synthetic bone substitute materials (to reduce the volume of donor material required), are used to fill the defect caused by the loosening of the existing

prosthesis. In order for the bone chips to form a stable foundation for the new prosthesis, they must be compacted together. The current surgical technique involves filling the bone defect that exists once the existing prosthesis has been removed with bone chips and then compacting the bone chips by repeated hammer blows to a "tamp" which is placed on top of the bone chips. The tamp serves to transmit the energy of the hammer blows to the bone chips to compact them together and to maintain an appropriately sized cavity within the graft formed by the bone chips to receive the new prosthesis. This process may be repeated several times until the defect is adequately filled and a graft of sufficient compactness is achieved. Sufficient compactness is judged at the time of surgery by the surgeon's experience and simple test methods, for example when the delivery of 10 controlled hammer blows does not cause any further progression of the tamp.

One difficulty with the impaction bone-grafting technique is that high forces generated during the hammering of the tamp are transmitted through the bone chips to the cortical shell of the host bone, which is often thin and weak in the revision situation, leading to high stresses and strains in the host bone and a concomitant increased risk of fracture. The fear of inducing fracture may cause the surgeon to under-compact the graft. Under-compacted graft results in an increased risk of migration of the new prosthesis when it is subjected to load. The risk of fracture presents a significant barrier to the wider adoption of impaction bone grafting by the orthopaedic surgical community.

An objective of the present invention is to improve the compaction of the bone chips used to produce the graft in the impaction bone grafting procedure with respect to the standard impaction grafting technique which uses hammer blows applied to the tamp. A further objective of the present invention is to reduce stresses and strains and subsequent risk of fracture during the graft compaction process, with respect to the standard impaction grafting technique.

The present inventors have realised that the bone graft produced by the impaction bone grafting procedure shares many characteristics with aggregate materials used in civil engineering applications such as soils, sands and ballasts used in the construction of railway beds and the sub-layers of roads. The behaviour of aggregates under load has been studied extensively by civil engineers and this knowledge ean be utilised to improve the mechanical characteristics of other aggregates such as bone graft. To produce the strongest aggregate, the following characteristics are required

• The aggregate should have a well graded particle size distribution (a mixture of bone chips of different sizes in appropriate proportions)

• The aggregate should have a low state of hydration

• The aggregate should be well compacted using an appropriate quantity of applied compaction effort, preferably by sequential compaction of layers of aggregate

• The aggregate should be adequately contained during compaction

Vibration applied to the aggregate is also commonly used in civil engineering applications in order to improve the compaction (packing together) of the aggregate particles and hence to increase aggregate compressive and shear strengths. Vibration applied to the aggregate, either on its own, in which case gravitational and surface energy forces cause compaction of the graft particles, or in conjunction with a additional applied loads, causes the aggregate particles to pack more efficiently.

If aggregate is subjected to vibration in an unconfmed space, the particles forming the aggregate are moved into a denser packing, which improves its shear strength. However, when the aggregate is saturated with a fluid phase, the interstitial fluid needs to be drained out as it becomes more densely packed. In a contained environment, such as in the space between the prosthesis and femur, the increase in fluid pressure generated by the vibration can lead to an increase in pressure in the fluid phase that will be exerted on the solid particles forming the aggregate. Under strong vibration, the fluid pressure can become large enough to push the solid particles apart, causing loss of contact. This phenomenon is known as liquefaction. Liquefaction can be avoided if adequate pressure relief is provided for the fluid phase.

The Invention

The present inventors has realised that vibration techniques can be applied to the compaction of bone graft material. Further, pressure relief is required for a fluid phase in the bone graft material during compaction, to prevent liquefaction of the graft material.

Accordingly, the present invention provides a bone graft compaction apparatus comprising a compacting tool for contacting and compacting bone graft material, a

vibrator for vibrating the compacting tool and means for relieving pressure in the bone graft material during compaction.

The present invention further provides a method of bone graft compaction, comprising contacting and compacting bone graft material with a compacting tool, vibrating the compacting tool and relieving pressure in fluid in the bone graft material during compaction.

Preferred and optional features of the invention will be described further below. The bone graft material used in the method of the invention or compacted with the apparatus of the present invention may comprise any conventional bone graft material, for example morsellised allograft material. The person skilled in the art will be aware of methods for preparing such material, for example by milling bone material such as femoral heads.

The compacting tool may comprise any suitable compacting tool. Preferably, it comprises a compacting surface for contacting bone graft material and a driving surface for contacting the vibrator.

Suitable compacting tools as known to the person skilled in the art for conventional impaction bone grafting may be used, for an example the type known as a tamp or phantom. For example, a model available under the mark "X-change" (from Stryker UK Limited) may be used. The compacting tool may be made of any suitable material, for example metal such as stainless steel.

The compacting tool may be provided with a longitudinally extending bore to allow it to be guided along a guide wire, in a manner known from the art of impaction bone grafting.

The frequency and amplitude should be selected to reduce the danger of fracture of the environment in which the bone graft material is being compacted.

For example, a vibration device available under the mark "Woodpecker" (Minnesota Bramstedt Surgical Inc) may be used. Such a device is controllable by controlling the pressure of operating fluid supplied. Preferably, the device is operated at a pressure in the range 1.2-5.0 bar, more preferably 2.0-3.0 bar.

The compacting tool and the vibrator may be rigidly coupled to transfer vibrations. There may be a fluid coupling or the vibrator may comprise an oscillating hammer which repeatedly impacts a drive surface of the compacting tool.

In the method of the invention, compaction may be carried out for a period of time necessary for satisfactory compaction to occur. This may be assessed, for example, by determining the point at which no further movement of the compacting tool with respect to a fixed reference occurs.

Preferably, the pressure relief means may be formed on a surface of or integral with the compacting tool. For example, channels may be formed in the surface of the compacting tool or holes may be formed in the surface of the compacting tool, communicating with a passage in the interior of the compacting tool. A pump may be provided for removing fluid from the bone graft material. Pressure relief holes preferably communicate with a longitudinally extending bore provided to allow the tool to be guided along a guide wire. This allows the holes to be provided in a simple manner and allows existing designs of tool to be adapted for the present invention.

Preferably, the vibrator is contacted with the compacting tool such that when vibration of the vibrator ceases, substantially no load is applied to the compacting tool. The only load applied may be the weight of the vibrator, which may be in the range 1- 5kg. In this way, the only load applied to the bone graft material is during the vibration. This has the advantage of being very controllable.

The present invention will be further described with reference to the accompanying drawings, in which:

Figure 1 is a sketch view of a vibration assisted impaction bone grafting device according to the present invention for a femur.

Figure 2 is a sketch of a vibration assisted impaction bone grafting device for the acetabulum.

Detailed Description of the Drawings

The invention is described here with respect to revision of the acetabular and femoral components of a failed hip replacement procedure, although the techniques could

equally be applied to the tibial, femoral or patella components of a knee replacement or any component of any other joint replacement.

With reference to Figure 1 which shows an apparatus suitable for use in revision of the femoral component of a hip replacement, the vibrator or vibration device (1) is coupled to the compacting tool or tamp (2) such that vibration generated by the vibrator (1) is transmitted to the tamp (2) and hence into the bone chips which will form the bone graft (5). The tamp (2) has a longitudinal hole which enables it to run over a guide wire (4). The tamp (2) incorporates a series of holes (3) which communicate with the longitudinal hole to allow pressure pulses in the fluid component of the bone graft (5) that accompany the vibration to be relieved.

Figure 2 shows a similar device designed for use in revision of the acetabular component of a hip replacement. The vibration device (6) is coupled to the tamp (7) such that vibration generated by (6) is transmitted to (7) and hence into the bone chips which will form the bone graft (9). The tamp (7) incorporates a series of holes (8) which communicate with the atmosphere to allow pressure pulses in the fluid component of the bone graft (9) that accompany the vibration to be relieved.