Orthopedics

The Role of Impaction Grafting: The When and How

Fares S. Haddad, BSc, MCh(Orth), FRCS(Orth); Faizal Rayan, D(Orth), MRCS(Ed)

Abstract

The challenges of acetabular revision include bone loss and poor bone biology. Favorable 10-year reports exist of uncemented acetabular revision using hemispherical uncemented sockets. In the presence of bone stock deficiency, adjunctive morsellized impaction bone grafting is a recognized means of restoring bone stock. We attempt to restore bone stock with impaction grafting beneath a cementless cup for contained defects, defects that can be rendered contained, and whenever there is capacity for rim fixation.

The technique of bone graft preparation is important. We use a mixture of bone milling and bone chips of various sizes. Morsellized allograft is inserted, packed, and/or reverse reamed into any defects. The reconstruction relies on the ability to gain biological fixation of the component to the underlying host bone. This requires intimate host bone contact and rigid implant stability. The fixation is therefore augmented with screws in all cases.

It is important to achieve host bone contact in a least part of the dome and posterior column. When this is possible, and particularly when there is a good rim fit, we have not found it absolutely necessary to have contact with host bone over >50% of the surface. Stability of primary fixation is a better predictor of outcome than volume of graft or percentage of host bone contact.

The advantages of bone grafting in acetabular reconstruction include the ability to restore bone stock, rebuild a normal hip center and hip biomechanics, and increase bone stock for future revisions.

Total hip replacement is one of the most successful orthopedic procedures for relieving pain and restoring hip function.1 In the United Kingdom, 66,000 primary hip replacements are performed every year. More than 10% are revision hip procedures,2 and of those 55% are for aseptic loosening. The revision rates for hip arthroplasty are 18% in United States, 11% in the United Kingdom, and 8% in Sweden.3 The increasing prevalence of arthroplasties in a younger population with high expectations and activity levels and greater life expectancy has led to a greater focus on the restoration of bone stock in revision surgery. Therefore, it is imperative to minimize bone loss at revision and to try to restore bone stock in younger patients whenever possible.

Revision of the failed acetabular component is a major undertaking that can be complicated by the loss of bone stock, a sclerotic bony bed, and soft tissue scarring and dysfunction.

The principles of acetabular reconstruction include the creation of a stable acetabular bed, secure prosthetic fixation with freedom of orientation, bony reconstitution, and the restoration of a normal hip center of rotation with acceptable biomechanics. The type and extent of acetabular bone loss determines the method of reconstruction. In our unit, we aim for a biological revision with bony ingrowth whenever possible and attempt to restore bone stock with impaction grafting beneath a cementless cup in the majority of acetabular revisions. Bone graft is not required in cases with minor bone loss where a hemispheral porous coated socket or trabecular metal socket will restore biomechanics. Alternate techniques, such as trabecular metal augments, antiprotrusio cages, and structural grafts are used for massive defects and when pelvic discontinuity is encountered.

Impaction bone grafting of the acetabulum in arthroplasty of the hip was described by Hastings and Parker4 in 1975 and subsequently popularized for revisions by Slooff et al.5 Bone graft is impacted into a contained defect or a defect that can be made contained to create a stable bed into which the prosthesis can be placed. Much data are available on the use of morsellized grafts with cemented sockets and cages.6-10 The procedure is technique-dependent, but good histology…

Abstract

The challenges of acetabular revision include bone loss and poor bone biology. Favorable 10-year reports exist of uncemented acetabular revision using hemispherical uncemented sockets. In the presence of bone stock deficiency, adjunctive morsellized impaction bone grafting is a recognized means of restoring bone stock. We attempt to restore bone stock with impaction grafting beneath a cementless cup for contained defects, defects that can be rendered contained, and whenever there is capacity for rim fixation.

The technique of bone graft preparation is important. We use a mixture of bone milling and bone chips of various sizes. Morsellized allograft is inserted, packed, and/or reverse reamed into any defects. The reconstruction relies on the ability to gain biological fixation of the component to the underlying host bone. This requires intimate host bone contact and rigid implant stability. The fixation is therefore augmented with screws in all cases.

It is important to achieve host bone contact in a least part of the dome and posterior column. When this is possible, and particularly when there is a good rim fit, we have not found it absolutely necessary to have contact with host bone over >50% of the surface. Stability of primary fixation is a better predictor of outcome than volume of graft or percentage of host bone contact.

The advantages of bone grafting in acetabular reconstruction include the ability to restore bone stock, rebuild a normal hip center and hip biomechanics, and increase bone stock for future revisions.

Total hip replacement is one of the most successful orthopedic procedures for relieving pain and restoring hip function.1 In the United Kingdom, 66,000 primary hip replacements are performed every year. More than 10% are revision hip procedures,2 and of those 55% are for aseptic loosening. The revision rates for hip arthroplasty are 18% in United States, 11% in the United Kingdom, and 8% in Sweden.3 The increasing prevalence of arthroplasties in a younger population with high expectations and activity levels and greater life expectancy has led to a greater focus on the restoration of bone stock in revision surgery. Therefore, it is imperative to minimize bone loss at revision and to try to restore bone stock in younger patients whenever possible.

The Principles of Acetabular Revision

Revision of the failed acetabular component is a major undertaking that can be complicated by the loss of bone stock, a sclerotic bony bed, and soft tissue scarring and dysfunction.

The principles of acetabular reconstruction include the creation of a stable acetabular bed, secure prosthetic fixation with freedom of orientation, bony reconstitution, and the restoration of a normal hip center of rotation with acceptable biomechanics. The type and extent of acetabular bone loss determines the method of reconstruction. In our unit, we aim for a biological revision with bony ingrowth whenever possible and attempt to restore bone stock with impaction grafting beneath a cementless cup in the majority of acetabular revisions. Bone graft is not required in cases with minor bone loss where a hemispheral porous coated socket or trabecular metal socket will restore biomechanics. Alternate techniques, such as trabecular metal augments, antiprotrusio cages, and structural grafts are used for massive defects and when pelvic discontinuity is encountered.

The Role of Bone Grafting in Acetabular Revision

Impaction bone grafting of the acetabulum in arthroplasty of the hip was described by Hastings and Parker4 in 1975 and subsequently popularized for revisions by Slooff et al.5 Bone graft is impacted into a contained defect or a defect that can be made contained to create a stable bed into which the prosthesis can be placed. Much data are available on the use of morsellized grafts with cemented sockets and cages.6-10 The procedure is technique-dependent, but good histology data show bony reconstitution over time.11-13

More recently, favorable results have been reported when morsellized grafts were used in combination with uncemented cups.14 Most cavitary defects and combined defects where a rim can be re-created are amenable to impacted morsellized bone grafting (Figures 1, 2). Acetabular bone grafts help to restore the center of rotation and contribute to the initial stability for the acetabular component. In the long-term, they restore pelvic bone stock for future revision surgery.15

As the amount of autologous bone graft available for an acetabular revision is limited, it is not usually a feasible option for revision surgery. The alternatives are allogeneic bone and bone substitutes. The latter has aroused considerable recent interest, but little clinical data have been published, and the current standard of care remains the use of allograft.16

Figure 1: Contained acetabular defect around a failed cemented acetabular component Figure 2: Revision with impaction grafting and a cementless hemispherical porous coated component fixed with screws

Figure 1: Contained acetabular defect around a failed cemented acetabular component. Figure 2: Revision with impaction grafting and a cementless hemispherical porous coated component fixed with screws.

Preoperative Assessment of the Bone Defect

Revision arthroplasty requires thorough clinical and radiographic evaluation to determine the degree of bone loss, quality of the remaining bone, presence of cortical continuity (containment), and absence of infection. Biplanar radiographic views are essential. Computed tomography and magnetic resonance imaging may be of value, particularly in the presence of massive bone loss. Careful preoperative assessment allows the selection of appropriate implants and graft. We favor the Paprosky classification of acetabular bone loss, which identifies the severity of bone loss, according to which the necessary fixation for a given bone loss pattern can be determined. According to the Paprosky classification system,17 there are 3 types of acetabular bone loss (type I, defect with an undistorted rim; type II, defect with distorted but intact rim; and type III, defect with nonsupportive rim). Preoperative radiographic criteria to predict the defect are superior migration of the hip center, ischial osteolysis, teardrop osteolysis, and implant position relative to Kohler line.

The Role of Uncemented Acetabular Cups

The deficient mechanical support and the sclerotic or nontrabecular nature of revision actabular defects are not ideal for cement fixation and have generated interest in cementless revision acetabular fixation.3,18 The durability and reliability of the fixation depends on the contact between viable bone and the implant and also on mechanical stability (movement <40-50 µm) for most isolated cavitary or segmental defects and for many combined deficiencies. Morsellized allograft is packed in using chips of varied size,19 and a combination of impaction and reverse reaming is used to create a hemisphere. The reconstruction relies on the ability to gain biological fixation of the component to the underlying host bone. This requires intimate host bone contact and rigid implant stability.

In the presence of bone stock deficiency, bone grafting of contained defects and the use of an uncemented, porous, coated component augmented by screw fixation is now a recognized method of acetabular revision.20

Favorable mid-term results have been reported with a high rate of graft incorporation when morsellized bone is used for contained acetabular defects.18,21-23 The minimum required host bone contact and the maximum amount of bone graft that may be tolerated without compromising stability and bone ingrowth in uncemented cups is not known. The importance of minimizing interposition of bone graft between host bone and implant has been emphasized previously.21,24

Radiostereometric studies correlate the use of morsellized bone grafting of contained defects with increased proximal migration of uncemented acetabular components.25 But neither the size of the acetabular defects nor the amount of morsellized bone graft had any influence on migration, and when hydroxyapatite-coated acetabular implants were used in combination with more forceful impaction of morsellized graft host bone contact of <50%, implant stability did not seem to be compromised.26 A combined use of bone allograft and hydroxyapatite-coated implants in an experimental setting has improved early fixation.27 Excellent early results were recently reported in a series of 50 acetabular revisions with hydroxyapatite-coated implants identical to ours, including 12 cases in which morsellized allograft was used.28

Garcia-Cimberto29 reported on a heterogeneous population of 65 revision acetabular cups in which bone grafts (a combination of morsellized and bulk grafts) and porous coated uncemented cups were used at a mean 8-year follow-up and noted an 11% re-revision rate and 22% poor clinical results. Seventy percent of cups had radiolucency in >1 DeLee and Charnley zones. In a subpopulation of cases with <50% host bone implant contact, however, they reported a 100% failure rate; they concluded that uncemented components are contraindicated in this group with <50% host bone contact. Etienne et al18 reported the revision of 108 hips for acetabular osteolysis with morsellized allograft and uncemented cups: 95% were clinically and radiologically successful at a mean follow-up of 85 months.

Palm et al20 suggested that oversizing the acetabular component by 2 mm is important to achieve maximum peripheral press-fit to whatever limited portion of the acetabular circumference is available. This seems to provide the necessary initial stability to allow bone ingrowths in combination with supplementary screw fixation, despite the large amount of allograft interposed between host and implant—even if the entire acetabular surface is covered by impacted bone graft.20 Insertion of the oversized cup probably denudes at least the peripheral parts while the graft in the central area undergoes some degree of compression by the implant.20

In our experience, it is important to achieve host bone contact in at least part of the dome and posterior column whenever possible. Additionally, when there is a good rim fit, we have not found it absolutely necessary to have contact with host bone over >50% of the surface.

Surgical Techniques and Tips

The indications in our center are for contained defects, defects that can be rendered contained, and any capacity for rim fixation. Thus, in terms of classifications, Paprosky types I and II defects are treated with impaction grafting and the insertion of a cementless socket.17 Our series includes cases with extensive acetabular bony defects all treated with impaction of morsellized allograft into the entire acetabular cavity.

Once the decision to attempt an uncemented reconstruction is made, the key steps are as follows: (1) Make a good exposure to define the defect. (2) Clear the membrane and prepare the surface to make it more likely to allow ingrowth by exposing healthy bone at the implant–bone interface as opposed to a sclerotic bony surface. To achieve this, use hemispherical reamers to prepare the acetabular cavity, and possibly even drill the sclerotic bone to get to some healthy bleeding tissue. Use sequentially larger reamers until 3-point contact is made with the ilium, ischium, and pubis. Acetabular reaming should be performed in the desired orientation of the final implant, with approximately 20° of anteversion and 40° of abduction (or lateral opening). Removing residual posterior column bone should be avoided. (3) The technique of bone graft preparation is important. We recommend using a mixture of bone milling and bone chips of multiple sizes ranging from slurry to 1-cm bone chips. In cases with insufficient bone graft, synthetic materials can also be used and have seen a good success rate with 50:50 mixtures. Insert, pack, and/or reverse ream the morsellized allograft into any cavitary defects. To apply this method to medial wall uncontained defects, place the graft onto the medial membrane or obturator internus muscle, and gently pack it down before inserting the uncemented acetabular component. Specially designed tamps can also be used to impact the graft. Either the reamer heads or trial cups can be used prior to choosing and inserting the definitive implant. (4) Next, achieve secure fixation. The fixation is augmented with screws in all cases. Cementless acetabular components with impaction grafting should not be used when the host biology does not allow for stability or bone ingrowth. This includes the severely osteopenic pelvis, pelvic osteonecrosis after irradiation, tumors, and metabolic bone disorders. They should also not be used in the presence of pelvic discontinuity unless the structure of the pelvic ring has been restored with a plate.

Future Perspectives

Osteoinductive materials such as bone morphogenic proteins can be used to promote bone formation.30,31 These have not yet been applied widely to revision hip surgery in clinical studies. The role of stem cells in regenerating bone has also generated a great deal of enthusiasm. An ovine model study conducted at our center has demonstrated significant bone growth in the group treated with stem cells.32 The impetus given by this study led us to apply stem cells onto the surface of acetabular components to improve ingrowth and bony contact. We were able to achieve better bone apposition, less fibrous tissue interposition, and greater bone formation both at the periphery and dome. We observed increased bone implant contact again in both areas, and 30% greater bone formation overall with the use of stem cells. This may prove a useful addition/adjunct to the work that Barrack et al33 have done in the past to try and improve acetabular fixation with bone graft.

Conclusion

The challenges of acetabular revision include access, soft tissue scarring and dysfunction, and, in particular, bone loss and poor bone biology. There are now favorable reports with a minimum 10-year follow-up of uncemented acetabular revision using hemispherical uncemented sockets in type I, II, and IIIA acetabular defects and in type IIIB defects; the early outcome of trabecular metal acetabular systems is also encouraging.3 We have found the adjunctive use of packed morsellized allograft in these cases favorable. The challenge of reconstituting the acetabulum depends on the degree and type of bone loss. The principles of maximizing host bone-implant contact and implant stability have born fruit in our experience with cementless revision. The advantages of bone grafting in acetabular reconstruction include the ability to restore bone stock, to rebuild a normal hip center and hip biomechanics, and to increase bone stock for future revisions.

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Authors

Dr Haddad is from the University College London & Princess Grace Hospitals, and Dr Rayan is from University College Hospitals, London, United Kingdom.

Dr Haddad receives research support from Smith & Nephew, Finsbury Orthopaedics, and Stryker. Dr Rayan has no relevant financial relationships to disclose.

Presented at Current Concepts in Joint Replacement 2008 Winter Meeting; December 10-13, 2008; Orlando, Florida.

Correspondence should be addressed to: Fares S. Haddad, BSc, MCh(Orth), FRCS(Orth), Princess Grace Hospital, 42 Nottingham Place, London W1U 5NY, United Kingdom.

DOI: 10.3928/01477447-20090728-19

10.3928/01477447-20090728-19

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