This technique is primarily used to manage huge acetabular bone defects in the posterior column, and a cementless acetabular cup is inserted into the upper part of the acetabulum with severe bone loss. The placement of the acetabular component was defined as the presence of hip center above a teardrop line connecting the superior aspect of the acetabulum greater than 35 mm17). When using this technique, the primary stability of the acetabular component and greater than 70% host bone contact with the cup should be obtained. The superior migration of the hip center causes no definitive problems with hip biomechanics, but the superolateral migration of the hip center needs to be avoided due to its adverse effects on hip biomechanics18). Since the superior migration of hip center may result in leg length shortening and abductor weakness leading to dislocation of the hip joint, the use of a long neck-head, a calcar replacement stem and a high offset stem is recommended. Excessive reaming into the posterior column should be avoided and all possible causes of impingement should be eliminated¹⁹20). This surgical technique is limitedly recommended and should be avoided if possible.

This surgical technique is used in progressive acetabular reconstruction in the presence of huge defects in acetabular bone stock. In this procedure, acetabular reconstruction is not completed via a single operation; instead, a bipolar cup is fixed at secondary surgery after restoration of bone stock and the incorporation of the bone graft into the host bone.

The surgical method can be used in AAOS type I, II, and III and Paprosky type 1, 2, and 3 bone defects, but is not desirable in AAOS type IV bone defects. A porous-coated cementless cup is typically used after chip bone grafting when the cup surface is in contact with more than 50% of the host acetabular bone and solid fixation of the acetabular cup can be attained with screws and alternatively, the use of a bipolar cup can be considered when firm fixation of the acetabular cup cannot be attained^21,2223). The primary concern with this surgical method is ensuring stable placement of the bipolar cup into the relatively healthy host bone through over-reaming of the acetabular rim. When bone loss is severe in the superomedial acetabular wall, it is important to prevent the postoperative medial displacement of the bipolar cup by using sufficient amounts of allograft.

A study of McFarland et al.24) showed that although clinical improvement was shown in 83% of cases, radiological outcomes at an average of 1.3 years of follow up revealed osteolysis around the acetabular component in most cases. Takatori et al.25) also note that the use of a bipolar cup should be limited since medial displacement of the cup of greater than 10 mm occurred in 40% of cases during an average follow-up of 7 years. This surgical technique has been rarely used in the clinical settings.

Structural allografts are often used in patients with AAOS type III and IV defects in the acetabular bone. This procedure fixes the structural bone at the site of acetabular bone defect and then the defect can be filled with autografts between the graft and host bones. Cemented and cementless cups and acetabular reinforcement ring are also used. Total acetabulum, distal femur, proximal tibia and femoral head allografts are commonly used as structural allograft materials. Usually, failure rates are high when structural grafts support more than 50% of the acetabular component26). Relatively good short-term results have been attained, but mid- and long-term failure rates range between 4% and 47%27). The major cause of failure is bone resorption during reformation of the grafted bone which leads to structural instability. When performing this procedure, the high long-term failure rates when structural grafts are attempting to support large areas of the acetabular component should be considered.

This surgical technique is commonly used for elderly patients with Paprosky type 2 and 3 structural defects in which greater than 50% host bone contact with the cup cannot be attained. The defects are filled with impacted moralized allografts and fixed with the acetabular reinforcement ring using screws, and a polyethylene liner 2 to 3 mm smaller than the acetabular reinforcement ring is fixed with cement. There are two types of Müller acetabular reinforcement rings: i) one that is fixed to the ilium alone with screws and ii) other reinforced types (e.g., Ganz and Octopus rings that are fixed to the ilium using screws and thus providing stability by a hook attached to cotyloid notch in the inferior aspect). Another ring type is Burch-Schneider antiprotrusio cage that is fixed to both the ilium and ischium28). The rate of short-term acetabular cup loosening is about 24%29), and the revision rates at mid- and long-term follow up are 20% and 44%, respectively30). Despite the use of a solid metal ring, Berry and Müller29) addressed problems encountered between the more flexible pelvic bone and a hard implant and stress shielding of the graft bone. The posterior column is fixed with a pelvic reconstruction plate in cases with pelvic incontinuity and the acetabular reinforcement ring is used for fixation. This technique is a commonly used surgical option, but should be carefully applied with accuracy. For this reason, the author of this study rarely uses this method.

In 1984, Slooff et al.31) used a modified morselized impaction allografting technique. In summary, the contained acetabular defect is impacted with an allograft (average size of 1 cm) and the segmental defect is covered with a metal mesh or a thin cancellous bone layer. Subsequently, cement is used directly onto the bone bed for fixation without using the acetabular reinforcement ring to allograft bone fragments. The key principles of this technique are: i) reconstruction of hip biomechanics by placing the cup on the anatomic teardrop, ii) impaction of the segmental defect with a metal wire mesh to achieve containment, iii) impaction of the cavitary defect with a morselized allograft to replace bone loss around implants, and iv) impaction of bone chips using bone cement to increase stability. The benefits of this surgical technique include: i) minimized polyethylene wear, ii) restoration of bone stock with satisfactory graft incorporation, and iii) stable interdigitation of cement with morselized grafts32).

This surgical method can be used in AAOS type I, II, and III and Paprosky type 1, 2, and 3 bone defects, but is not desirable in AAOS type IV defects. When performing this technique, the following cautions should be carefully considered: i) massive segmental defects should be converted to cavitary defects using metal meshes, ii) small-sized chip bone grafts are impacted into the site of the cavitary defect, and iii) the polyethylene acetabular cup is cemented onto the graft.

A wide variety of radiological and clinical outcomes relating to this technique have been reported. Previous studies that introduced this surgical technique have reported favorable results. Studies on aseptic loosening of the acetabular component by Trumm et al.33), Highcock et al.34), and Sloof et al.35) demonstrated a low re-revision rate of 4% to 6%. Buttaro et al.36) documented a success rate of 90% in 23 hips at 3-year follow-up. Comba et al.37) obtained favorable results in 96% of 142 hips at 4-year follow-up. Schreurs et al.38) note that cemented acetabular cup fixation is considered a good surgical option because of a survival rate of 85% at an average follow-up of 12 years. On the contrary, Jasty and Harris39) reported a failure rate of 75% at 6-year follow-up, and Pellicci et al.40) and Kavanagh et al.41) also obtained unsatisfactory results with a re-revision rate of 22%, complication rate for sepsis at 2.5%, recurrent dislocation at 4%, aseptic loosening at 16% and others. Nevertheless, this technique is considered fairly safe.

This is a widely used surgical technique that restores the anatomic hip center and facilitates incorporation of the bone graft. To use a cementless cup, ensuring as much contact as possible with healthy host bone is critical. Although the extent of viable host bone contact with a cementless cup remains controversial, it is generally considered that the cup surface should make contact with at least 50% of healthy host bone in cavitary defects. In addition, it is desirable to fix the cup by inserting extra-long screws from different directions. Greater than 70% host bone contact with a cementless cup is commonly recommended in segmental defects⁴²43).

This surgical method can be used in AAOS type I, II, and III and Paprosky type 1, 2, and 3 bone defects, but is not allowed in AAOS type IV defects and patients with Paget's disease, metabolic bone disease (e.g., acetabular necrosis and bone tumor)42). The success of this surgery is considerably affected by the preservation of the posterior column. Moreover, solid impaction of morselized bone chips is important and initial stability needs to be attained by placement of the cup with the acetabular rim and acetabular floor on the remaining host bone.

Multiple mid- and long-term studies report relatively good clinical results in fixation with a cementless vs. cement cup. Silverton et al.44) documented a radiological failure rate of 7% in a study with a median follow-up period of 8.3 years. In a study by Sun et al.45), radiological findings revealed osteolysis in 24.6% of patients at an average follow-up of 8.2 years, but the survival rate of implants was 92.1%. Leopold et al.46) noted a survival rate of 98% and a noninfectious acetabular loosening rate of 1.8% at a median follow-up of 10.5 years. Rosenberg47) reported a survival rate of 84% and no revisions due to loosening at an average follow-up of 11 years after revision in 138 hips. The author of this study prefers the surgical method using morselized impaction allograft and a cementless cup and intends to more clearly delineate this procedure in the future.

Trabecular metal augmentation has been introduced to improve biological fixation rather than mechanical fixation and is used in revision when there is clearly a smaller contact surface between the implant and host bone due to osteolysis. This technique facilitates bone ingrowth by using tantalum to fill the bone defect and trabecular metal material can be an alternative to structural allograft. Since tantalum has properties of high volumetric porosity and low modulus of elasticity and exhibits high coefficient of friction, this metal can ensure primary implant stability48). Furthermore, this technique is a simple and quick procedure, and can achieve biological fixation with bone ingrowth without the risk of bone resorption after grafting.

This surgical method can be used in Paprosky type 3 defects, type 3A with severe bone loss in the superior aspect of the acetabulum and less than 50% host bone contact and type 3B associated with 3A defects or pelvic discontinuity.

Siegmeth et al.49) performed acetabular revisions using trabecular metal augments and trabecular metal cups in 34 cases, and stable fixation was achieved in 32 requiring no additional surgery; re-revision was required in only two cases with more than 2 years of follow-up. Other authors obtained comparable results and suggest that this technique is a surgical option for bone ingrowth around the cup when contact surface between the cup and host bone is small and firm screw fixation is impossible⁵⁰51). Boscainos et al.52) report more than 32-month follow-up results of 14 patients who underwent revision using the trabecular metal cup-cage construct, and all patients gained stable implant fixation; only two patients underwent re-revision due to dislocation.

However, since the clinical outcomes of trabecular metal augments are insufficient due to short-term follow-up, further long-term investigations are warranted to improve long-term clinical results (e.g., stability, wear debris between the cup and metal augments, fatigue failure, the amount of the host bone required in revision, and difficult recovery).

A cage-in-cup technique is a recent surgical option for acetabular fixation using the cup-cage construct. In this method, a second-generation porous-coated cup is used for pelvic fixation. When implant stability is weak, an acetabular cage can be placed into the superior portion of the cup and fixed to the ilium and ischium by screws. Acetabular revision arthroplasty using a cup-cage construct is a useful technique when managing Paprosky type 3A and 3B defects and pelvic discontinuity. In a case report by Bellester Alfaro and Sueiro Fernádez53), no failures were observed among 5 patients who underwent revision using cup-cage constructs and trabecular metal augments (an average follow-up of 26 months). Further studies are warranted to investigate this rarely used technique and only explored with short-term follow-up in Korea.

The author of this study prefers a transgluteal approach, but concomitantly uses trans-trochanteric osteotomy, if necessary. After exposure of the surgical site, a loose acetabular component, osteolytic soft tissues and all bone cements are completely removed. After observation of the defect area, reaming is continued using progressively larger-diameter reamers. Reaming is carefully carried out to ensure maximum stability without damaging structures within the periacetabular region. Sufficient medialization of the acetabular component is obtained by conserving as much contact with the original bone bed as possible. In particular, the contact area between the cup and host bone should be maximized by progressively increasing reamer diameters as tolerable to optimize contact pressure in the anterior, inferior and posterior areas. Allografts used in implantation are harvested from the femoral head, frozen and stored under −80℃ for more than 6 months in a tissue bank after being collected from a patient with femoral neck fracture. Bacterial cultures are done before allografts are frozen and stored, and screening tests are performed on each donor. Cartilage and cortical bone are removed from the allograft, and separated cancellous bone is cut into about 1 cm using bone scissors. To minimize immune response, allografts are repeatedly washed with saline solution using pulsatile lavage to eliminate as much fat and blood as possible and then dried with a skin towel. Dried morselized impaction allografts are mixed with the patient's blood. Morselized bone grafts are impacted into the defect using impactors with reverse reaming (Fig. 1). Proper reaming allows satisfactory results using morselized impaction allograft despite major bone loss affecting more than 60% of the acetabulum. A hemispherical jumbo cup larger than a reamer by 2 mm is press-fit and firmly fixed to the superior portion using 4 to 7 cancellous screws to ensure bony ingrowth (Fig. 2). When severe bone loss is managed with morselized impaction allograft alone, a jumbo cup with a diameter of 66 to 74 mm is commonly used.

The author prefers allograft in mild bone defects unless a bone graft is required. Adequate fixation can be achieved in some cases when morselized impaction allograft is applied to a Paprosky type 3B defect with more than 60% of bone loss. However, when sufficient fixation cannot be attained due to severe segmental defect in the medial wall and bone stock first, one of the two techniques used for early fixation is used. The first tricortical bone graft which is harvested from the iliac tuberosity the most abundant source of cancellous bone via an incision of 5 to 7 cm, and then fixed to the superior acetabular margin using 2 to 3 screws. This technique is similar to slotted acetabular augmentation described by Staheli54). In addition, the remaining bone defect is packed with morselized allograft and cancellous autograft reaming is applied to help impact the graft (Fig. 3). Secondly, initial fixation may be obtained via adequate reaming with structural allograft from the femoral head, and the remaining defect is well designed with femoral head allograft. Femoral head structural graft is fixed with 2 to 3 cancellous screws. A minimum compressive load to the graft bone needs to be generated, and as much of the load as possible should be in contact with the remaining acetabular rim. Congruency of the acetabulum is ensured by packing morselized allograft around the structural allograft (Fig. 4). Despite more than 60% of bone loss between the boundary of Paprosky type 3A and 3B defects, the author's revision technique can be performed if pelvic discontinuity is not severe, the posterior column and acetabular dome remain after maximum reaming, or a jumbo cup can be placed on the acetabular margin, despite a discontinuity of the acetabular rim (Fig. 2). Reaming the exact amount of the diameter of the acetabular component should be ensured to obtain satisfactory fixation.

Passive hip joint and knee joint exercises are allowed from the second to third postoperative day, and partial weight bearing ambulation is normally begun from 3–4 days to 6 weeks after surgery depending on rigidity of fixation. Partial weight bearing gait with heavier weight load is recommended at the sixth postoperative week and full weight bearing gait is allowed three months after surgery. However, non-weight bearing ambulation can be performed in cases with excessive allograft implantation by considering patient's age and systemic status.

With respect to ossification when using a cementless acetabular cup and morselized impaction allograft, first, bony ingrowth in the contact surface with autograft encourages fixation with autograft. Second, host bone-derived new blood vessels and osteoblasts flow into a dead space in the contact area with morselized impaction allograft, and progressive ossification enables fixation with allograft as a scaffold structure. Sloof et al.31) proved bony ingrowth from a cementless acetabular cup in contact with allograft based on the results of laboratory animal tests. Although elucidated in a study of Lee et al.^15,55,5657), bone ingrowth appears to occur indirectly without a radiolucent line as shown in the author's long-term follow-up.

The advantages of the author's technique are the ability to achieve greater early fixation and incorporation using press-fit technique and morselized impaction bone compared to previous techniques. Avoiding direct weight load to the structural allograft minimizes the risk of collapse and this facilitates bony ingrowth, leading to restoration of the segmental defect.

The procedure, as stated by Lee et al.15), has been used since the early 1990s. Instead of fixation loss, bony union and graft bone changes in hip joints at a minimum follow-up period of 10 years (range, 10–20 years 2 months) after morselized impacted allograft into acetabular bone defect were reported in 2012. This previous study reviewed 98 hips (93 patients) followed up for at least 10 years (range, 10–20 years 2 months) to examine: i) new bone formation, ii) changes in the radiolucent zone between allograft and cup size, iii) changes in the allograft margin, and iv) formation of trabecular bone by comparing enlarged anteroposterior and lateral views on radiographs taken immediately after surgery and at final follow-up. Since determining incorporation between the host bone and graft bone is challenging, incorporation is defined as the finding of newly formed trabecular bone and radiographic disappearance of borders between the host bone and graft bone on follow-up anteroposterior views.

According to the radiographic results at final follow-up, a radiolucent zone between the host bone and graft bone was observed in 3 cases and periacetabular component in 12 cases, but the width of all radiolucent zones was less than 2 mm. When comparing radiographs taken on regular follow-ups, radiolucency of the graft bone was increased in 38 cases and decreased in 58 cases on follow-up radiographs taken between 3rd and 6th postoperative months. Newly formed trabecular bone and incorporation between the host bone and graft bone were confirmed in all cases, excluding 15 hips. There were no complications associated with injury to the nerves including the sciatic nerve, and no revision surgery due to deep infection. Although postoperative dislocation occurred in 1 hip, this was managed with an abduction brace after closed reduction. Reformation of the graft bone seems to be affected by patterns in stress change. Therefore, the author's technique with structural autograft and a cementless cup is suggested as a good surgical option for acetabular revision arthroplasty assuming accurate surgical procedures are applied.

In 2004, Lee et al.15) reported outcomes of morselized allograft in revision for management of acetabular bone defect and reviewed 77 cases (81 hips) with acetabular revision using morselized allograft bone by reviewing the anteroposterior and lateral views on radiographs taken at final follow-up (range, 6–12 years and 10 months). Fixation was well maintained in 30 out of 31 cases. Compared to immediately after surgery, radiodensity increased in 32 cases and decreased in 48 cases. A radiolucent zone between the host bone and graft bone was observed in 2 cases and around the acetabular component in 9 cases; all radiolucent zones had a width of less than 2 mm. Moreover, reformation of the medial graft bone was observed with acetabular bone resorption.

In a study published in 2011, Lee et al.56) reported radiographic and clinical results in 62 of 71 hips that underwent surgery using morselized impaction allograft and a cementless cup based on radiographic images and Harris hip scores at a minimum of 10 years (range, 10–14 years and 8 months) from 1992 to 2000. The mean Harris hip score was 92 at final follow-up and re-revision was done in 3 cases. There were no injuries to the blood vessels and nerves including sciatic nerve palsy, and two patients with recurrent dislocation within the first postoperative week were managed with an abduction brace. The 12-year survival rate was 95.8%. These outcomes demonstrate that satisfactory long-term results can be obtained using the author's surgical technique.