Severe post-traumatic and post-arthroplasty acetabular defects: beyond Existing classifications

Introduction

Bone defect patterns in post-acetabular fracture malunions are different from those in revision hip arthroplasty. Bone defects due to aseptic loosening of cemented or cementless arthroplasty are due to a gradual process that occurs over a longer period of time. These bone defects, seen after fractures, are acute and are determined mainly by the initial fracture pattern and initial surgical intervention.

Due to excessive bone loss, orthopaedic surgeons seldom see severe late post-traumatic or post-arthroplasty acetabular defects which are not easily classified into existing systems. When such acetabular defects are treated with conventional reconstruction techniques, standards for anatomical restoration of center of rotation and acetabular offset are not often met.

Acetabular defects due to malunion or nonunion of old acetabular fractures are rare due to recent improvements in implants and surgical skills. However, when they do appear, their management is problematic.[1] Sen RK, Mukhopadhyay R, Pattanshetti V, Saini G, Tripathy SK, Sethy SS, Sharma SK. A New Classification System for Acetabular Bone Defect Evaluation in Posttraumatic Acetabular Nonunion and Malunion. Indian J Orthop. 2022 Jun 27;56(9):1601-1612

Chronic pelvic discontinuity (PD), or pelvis dissociation, in total hip arthroplasty (THA) is the loss of structural continuity (fracture) between the superior and inferior parts of the pelvis. It progresses through the anterior and posterior columns of the acetabulum so that the superior bone structures of the pelvis are completely dissociated from the inferior ones. Biology, mechanics and thus the healing potential of PD is similar to atrophic nonunions. It is relatively uncommon, but its incidence is increasing due to an increasing number of primary and multi-revision THAs.[2], Schwarzkopf R, Ihn HE, Ries MD. Pelvic discontinuity: modern techniques and outcomes for treating pelvic disassociation. Hip Int. 2015; 25(4):368-74.[3] Babis GC, Nikolaou VS. Pelvic discontinuity: a challenge to overcome. EFORT Open Rev. 2021; 6(6):459-471.

Classification

A classification system should accurately describe the problem, assist pre-operative evaluation and surgical pre-operative planning, and should predict outcomes. The recent Sen et al classification of late post-traumatic acetabular defects clearly reveals the patterns of post-traumatic nonunions and malunions.[1] Sen RK, Mukhopadhyay R, Pattanshetti V, Saini G, Tripathy SK, Sethy SS, Sharma SK. A New Classification System for Acetabular Bone Defect Evaluation in Posttraumatic Acetabular Nonunion and Malunion. Indian J Orthop. 2022 Jun 27;56(9):1601-1612 In the commonly used Paprosky classification, PD is often associated with IIIB and infrequently with IIC and IIIA bone defects.[4] Paprosky WG, Perona PG, Lawrence JM. Acetabular defect classification and surgical reconstruction in revision arthroplasty. A 6 year follow up evaluation. J Arthroplasty. 1994;9(1)33-34. However, the Paprosky classification fails to describe post-traumatic defects, and occasionally, orthopaedic surgeons encounter other acetabular bone defects which are not described by existing classification systems (Figure 1).

Diagnosis

As a general principle, first ruling out infection is mandatory in all cases of acetabular defects.

Conventional anteroposterior and lateral radiographs may initially be used for pre-operative planning for both post-traumatic and post-arthroplasty severe bone defects. PD should be suspected in cases with excessive osteolysis or fracture. When Kohler’s line and the tear drop is violated, and if moderate to severe ischial lysis is observed, and the cup and femoral head have migrated vertically more than 3 cm, one can safely suspect PD.[4], Paprosky WG, Perona PG, Lawrence JM. Acetabular defect classification and surgical reconstruction in revision arthroplasty. A 6 year follow up evaluation. J Arthroplasty. 1994;9(1)33-34.[5] Sporer SM, Paprosky WG. Acetabular revision using a trabecular metal acetabular component for severe acetabular bone loss associated with pelvic discontinuity. J Arthroplasty. 2006; 21(6)(S):87-90. However, the diagnosis of both post-traumatic malunion or nonunion and the diagnosis of PD in standard imaging views are difficult due to the fact that the implants obstruct full visibility of bone defects, especially when the posterior column is involved.[2], Schwarzkopf R, Ihn HE, Ries MD. Pelvic discontinuity: modern techniques and outcomes for treating pelvic disassociation. Hip Int. 2015; 25(4):368-74.[3] Babis GC, Nikolaou VS. Pelvic discontinuity: a challenge to overcome. EFORT Open Rev. 2021; 6(6):459-471. The use of a helical CT-scan and, more recently, the use of CT-scan thin slices in three planes, in combination with metal artifact reduction techniques, have improved diagnostic accuracy (Figure 2).[6], PuriL, Wixson RL, Stern SH, Kohli J, Hendrix RW, Stulberg SD. Use of helical computed tomography for the assessment of acetabular osteolysis after total hipa arthroplasty. J Bone J Joint Surg Am. 2002; 84(4):609-614.[7] Wellenberg RHH, Hakvoort ET, Slump CH, Boosma MF, MAAs M, Streekstra GJ. Metal artifact reduction techniques in musculoskeletal imaging. Eyr J Radiol. 2018; 107:60-69. Occasionally, a pre-operative CT angiogram is performed, when the removal of an intra-pelvic acetabular cup is planned. Despite technological improvements, one has to understand that resulting bone loss is usually greater than that which is initially anticipated during pre-operative planning.

Intra-operatively, nonunion and a PD diagnosis are made by applying stress, using a Cobb elevator, to the inferior hemipelvis (anterior and posterior column) in an anteroposterior direction and observing movement between the superior and posterior parts of the acetabulum.

Management strategies

The treatment goals are the anatomical restoration of the center of rotation, acetabular offset and the achievement of long term interface stability using revision arthroplasty acetabular implants.

On order to achieve this, surgeons face several challenges such as the classification of bone defects, the selection of appropriate implants, choice of surgical technique and possible bone grafts. There is also a high incidence of intra- and post-operative complications, post-operative mechanical failures and an absence of criteria for evaluating clinical and radiological outcomes.[2], Schwarzkopf R, Ihn HE, Ries MD. Pelvic discontinuity: modern techniques and outcomes for treating pelvic disassociation. Hip Int. 2015; 25(4):368-74.[3] Babis GC, Nikolaou VS. Pelvic discontinuity: a challenge to overcome. EFORT Open Rev. 2021; 6(6):459-471. Surgery should be carefully planned and the orthopaedic team should be prepared for the unexpected, changing techniques and using various implants and grafts as appropriate. Most surgeons prefer the posterior approach which allows for wider exposure and possible extension. Extended trochanteric osteotomy, when indicated, allows for even wider exposure and acetabular visibility.

Severe post-operative defects. Principles of management and outcomes of specific patterns are not clearly reported in the literature. Based on the Sen clacification,[1] Sen RK, Mukhopadhyay R, Pattanshetti V, Saini G, Tripathy SK, Sethy SS, Sharma SK. A New Classification System for Acetabular Bone Defect Evaluation in Posttraumatic Acetabular Nonunion and Malunion. Indian J Orthop. 2022 Jun 27;56(9):1601-1612 types 1, 2, 3-A and 5 can be treated with surgical techniques used in revision THA surgery. For severe defects (such as 3-B, 4-A and 4-B), the restoration of hip center of rotation, cup offset, inclination and anterversion and hip stability can be achived only with custom-made implants (Figures 3 and 4).

Chronic PD and unclassified severe defects. The goals of reconstructive surgery are the following: identification and confirmation of the problem; removal of PD scar tissue leaving clean bone edges; re-establishment of the continuity of the superior and inferior hemipelvis with implants and bone graft when necessary; achievement of stability of the reconstruction, and, if possible, restoration of bone stock.[2] Schwarzkopf R, Ihn HE, Ries MD. Pelvic discontinuity: modern techniques and outcomes for treating pelvic disassociation. Hip Int. 2015; 25(4):368-74.

Several reconstruction techniques are available:

Cages and Rings with graft. Historically, massive bulk allograft, used with a cemented liner in the setting of severe bone defects had a 50% failure rate.[8] Garbuz D, Morsi E, Gross AE. Revision of the acetabular component of a total hip arthroplasty with a massive structural allograft. Study with a minimum five years follow up. J Bone Joint Surg Am. 1996; 78:693-697. In the past, reconstruction cages and rings have also been used extensively to reconstruct severe acetabular bone defects and PD. Structural or morsellised bone allografts were used in order to fill the acetabular defect and then a cage or ring was used to protect the grafted area until the allograft was incorporated by creeping substitution. Acetabular components were then cemented in an appropriate orientation. Several manufacturers have developed various ilioiscial (onlay or inlay) or non-ilioischial spanning cages and rings. However, despite reports of early satisfactory outcomes, many of these devices were later withdrawn due to subsequent mid- and long-term failures (Figure 5a).[9], Regis D, Sandri A, Bonetti I, Bortolami O, Bartolozzi P. A minimum of 10 year follow up of the Burch-Schneider cage and bulk allografts for the revision of pelvic discontinuity. J Arthroplasty. 2012; 27:1057-1063.e1.[10] Abolghasemian M, Tangsaraporn S, Drexler M, Barbuto R, Backstein D, Safir O, Kuzyk P, Gross A. The challenge of pelvic discontinuity: cup-cage reconstruction does better than conventional cages in mid-term. J Bone Joint Surg Br. 2014; 96-B(2):195-200.

Internal fixation with acetabular reconstruction. Plate and multiple screw fixation are used in combination with cementless acetabular cups in order to stabilise a post-traumatic acetabular nonunion or a PD. Single plating of either the anterior or posterior column or double plating of both the anterior and posterior column through a combined ilioinguinal and posterior approach have been suggested.[2], Schwarzkopf R, Ihn HE, Ries MD. Pelvic discontinuity: modern techniques and outcomes for treating pelvic disassociation. Hip Int. 2015; 25(4):368-74.[11] Villanueva M, Rios-Luna A, Pereiro De Lamo J, Fehandez-Saddi H, Bostrom MP. A review of the treatment of pelvic discontinuity. HHS J. 2008;4(2):494-502.

Acetabular distraction with cementless acetabular cup. Following the removal of scar tissue and mobilisation of the discontinuity site (often a difficult task) laminar spreaders or Cobb elevators are used in order to pull apart and expand the defect before the implantation of a cementless acetabular component. The distracted PD bone edges then apply compression forces to the implant, improving reconstruction stability.[12], Brown NM, Hellman M, Haughom BH, ShahRP, Sporer SM, Paprosky WG. Acetabular distraction: an alternative approach to pelvic discontinuity in failed total hip replacement. J Bone Joint Surg Br. 2014; 96(11):73-77.[13] Sporer SM, Bottros JJ, Hulst JB, Kancherla VK, Moric M, Paprosky WG. Acetabular distraction: an alternative for severe defects with chronic discontinuity? Clin Orthop Relat Res. 2012;470:3156-3163.

Tantalum cementless acetabular cups with augments. TM and related (e.g. Cription) technology acetabular components fixed with multiple holes superiorly and inferiorly (ilium, pubic and ischial rami) in combination with augments have recently been used in severe acetabular defects and chronic PD, especially in those with good bone stock (Figure 5b).[14] Batuyong ED, Brock HS, Thiruvengadam N, Maloney WJ, Goodman SB, Huddleston JI. Outcome of porous tantalum acetabular components for Paprosky type III and IV defects. J Arthroplsty. 2014; 28: 1318-1322. Implants made of highly porous structural materials (e.g. Tantalum and Cription) have demonstrated an improved friction coefficient for initial stability and better bony ingrowth for late stability. Early and mid-term outcomes are encouraging, allowing for extending the indications for their use (Figure 6).[15] Jenkins DR, Odland AN, Sierra RJ, Janssen AD, Lewallen DG. Minimum dive year outcomes with porous tantalum acetabular cup and augment construct in complex revision total hip arthroplasty. J Bone Joint Surg Am. 2017; 99:e49.

Cup and cage construct. This technique involves the placement of a highly porous Tantalum jumbo acetabular cell or cup against the host bone, bridging the acetabular defect. On the top of the cell, a special inlay cage is placed which spans the defect and is fixed into the ilium and ischium.[16] Hanssen AD, Lewallen DG. Modular acetabular augments: composite void fillers. Orthopaedics. 2005; 28:971-972. Tantalum augments are also used when necessary. “Half” cup cage reconstruction (removing the distal flange from the “full” cup-cage construct in order to address technical problems with its distal insertion) has also been introduced.[17] Sculco PK, Ledford CK, Hanssen AD, Abdel MP, Lewallen DG. The evolution of the cup-cage technique for major acetabular defects: ull and half cup-cage reconstruction. J Bone Joint Surg Am. 2017; 99:1104-1110. Mid-term outcomes are favourable and this technique has gained in popularity.[18], Amenabar T, Rahman WA, Hetaimish BM, Kuzyk PR, Safir OA, Gross AE. Promising mid term results wit a cup and cage construct for large acetabular defects and pelvic discontinuity. Clin Orthop relat Res. 2016; 474:408-414.[19] Conan S, Dunscan CP, Masri BA, Garbuz DS. The cup-cage reconstruction for pelvic discontinuity has encouraging patient satisfaction and functional outcome at median 6 years follow up. Hip Int. 2017; 27(5):509-513.

Triflange and tumour like custom made acetabular impants. Modern technology allows the production of personalised implants for special conditions. These type of implants are custom made, porous coated titanium implants and are considered the last therapeutic salvage option, before excisional arthroplasty, for severe acetabular defects and PD. The implants are designed and manufactured based on pelvic models created by thin-slice pelvic 3-D CT-scans with metal subtraction software (Figures 7 and 8). Early and mid-term outcomes are encouraging but premium cost and length of production time limit their use.[20], DeBoer DK, Christie MJ, Brinson MF, Morrison JC. Revision total hip arthroplasty for pelvic discontinuity. J Bone Joint Surg Am. 2007; 89(4):870-876.[21], Taunton MJ, Ferhing TK, Edwards P, Bersasek T, Holt GE, Christie MJ. Pelvic discontinuity treated with custom triflange component: a reliable option. Clin Orthop Relat res. 2012; 470(2):428-434.[22] Zhang Y, Gao Z, Zhang B, Du Y, Ma H, Tang Y, Liu Y, Zhou Y. The application of custom-made 3D-printed titanium augments designed through surgical simulation for severe bone defects in complex revision total hip arthroplasty. J Orthop Traumatol. 2022; 6:23(1):37.

Concerns for the use of custom made implants are related to the accuracy of the image based technique used for their production, the accuracy of the implant 3D fitting and implantation, the appropriate surgical approach and exposure and the manufacturing and biology of the implant-bone interface.[23], Hothi H, Henckel J, Bergiers S, Di Laura A, Schlueter-Brust K, Hart A. The analysis of defects in custom 3D-printed acetabular cups: A comparative study of commercially available implants from six manufacturers. J Orthop Res. 2023; 41(7):1505-1516.[24], Di Laura A, Henckel J, Wescott R, Hothi H, Hart AJ. The effect of metal artefact on the design of custom 3D printed acetabular implants. 3D Print Med. 2020; 26:6(1):23. [25] Gruber MS, Jesenko M, Burghuber J, Hochreiter J, Ritschl P, Ortmaier R. Functional and radiological outcomes after treatment with custom-made acetabular components in patients with Paprosky type 3 acetabular defects: short-term results. BMC Musculoskelet Disord. 2020; 10:21(1):835.

Clinical outcomes. A literature search shows that comparisons of the results of these techniques are difficult. Clinical reports are inconclusive because they present early to mid-term outcomes in a small number of patients with dissimilar bone defects, treated with different strategies and implants.2,3 However, all authors agree that late post-traumatic malunions and nonunions, severe acetabular defects and PD are challenging clinical entities in adult hip reconstructive surgery, and despite the fact that contemporary surgical techniques are promising, the cost is high and results are complicated by a high incidence of infection and dislocation.