Voshicage Revision total hip arthroplasty: addressing acetabular bone loss A comparison of the reliability and validity of bone stock loss classification systems used for revision hip surgery. Which acetabular bone defect classification and treatment option best describes this scenario? How important is this topic clqssification clinical practice? Reconstruction of severe acetabular defects with associated pelvic discontinuity is a challenging problem. The majority of acetabular revisions can be performed with an uncemented hemispherical cup.
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The ultimate goal of revision acetabular reconstruction should be to obtain stable fixation and restore the hip center[ 5 ]. Previous studies have demonstrated the relationship between polyethylene wear and progressive osteolysis with compromised bone stock[ 9 ]. Liner exchange with highly cross-linked polyethylene has been shown to decrease average wear rates significantly[ 10 ]. To justify isolated liner exchange, the modular metallic shell should be well-fixed and appropriately oriented[ 11 ].
This should be evaluated both pre-operatively and intra-operatively. Once the stability of the acetabular prosthesis is confirmed and liner exchange is contemplated, it is then important to consider the adequacy of the locking mechanism between the liner and the metallic shell. If the locking mechanism is compromised, one may consider cementing a new liner into the fixed metallic shell to prevent micromotion between the two surfaces for primary fixation.
The clinical track-record and historical performance of the implant should be considered along with the available liner and head size options offered by that particular component. Hemisphere reconstruction Historically, treatment options for acetabular component instability or malposition included use of cemented acetabular all-polyethylene prostheses implanted with the same techniques that had been employed for the primary arthroplasty procedure.
The results of the cemented revision procedures were poor, resulting from mechanical failure secondary to poor cement interdigitation and fixation leading to excessive micromotion within the acetabular bed[ 12 ]. In contrast, hemispheric metal cups with porous coating and associated techniques have been developed that encourage bone in-growth and held the promise of durable biologic fixation.
Cementess hemispherical porous-coated implants are the most commonly used implants for acetabular reconstruction in North America. With supportive and viable host bone and a reliable in growth surface, the cups address most revision problems encountered.
Initial stability is provided with a press-fit and screw fixation. Cavitary defects are addressed with morselized bone graft. These components are acceptable for patients who have not shown evidence of hip center migration or significant pelvic discontinuity Paprosky types I, IIA and IIB , which can be assessed pre-operatively as well as intra-operatively[ 5 ]. Internal fixation with screws is also advocated to supplement the in-growth of the press-fit component.
When there are focal superior segmental or cavitary defects identified at that time of revision, modular metallic augments, structural allograft, or morselized impacted allograft[ 13 ] may be added to supplement the acetabular bed.
Care must be taken to maintain the appropriate orientation of the revision cup despite the presence of augments in the dome. Park et al[ 14 ] recently published long-term data from their cohort of patients who underwent revision hip arthroplasty with use of a cememntlessacetabular shell.
Jumbo cups following allografting for focal defects also have a role in acetabular revision surgery. There is no universally-acceted definition of what diameter defined the jumbo cup. Jumbo cups are loosely defined by the ratio of component size to the pelvis and the hip joint, as compared to the size of the original implant[ 15 ].
These jumbo cups offer numerous advantages in regards to maximizing the contact area between the cup and host bone when a larger reamer is necessary to establish rim contact. The larger components can also accommodate larger femoral heads, reducing the rate of dislocation.
A large mismatch between a large shell and a small femoral head may increase the rate of impingement and reduce the soft tissue constraints to dislocation and is to be avoided. Another potential disadvantage with the jumbo cup comes with displacement of the femoral head hip center into a lateral inferior position, which has been reported.
High hip center placement of an uncementedacetabular hemispheric component is another option when there is a defect in the superolateral dome or posterior column that precludes the standard placement of a hemispheric shell in a more anatomic location. To accommodate the defect, the shell is placed in a more superior position. Accordingly, it is often necessary to do concomitant procedures at the time of revision to ensure that soft tissue tension and appropriate leg lengths are restored.
As superior placement of the hip center can also be associated with lateralization of the component, there have been some reports of increased dislocation or loosening rates with high hip center placement[ 16 ]. A high hip center is also disadvantageous from a biomechanical standpoint and will typically result in a limp. Hip stability may be compromised due to the small head size and bony impingement. A traditional contraindication to use of a cementless hemisphere revision component was recent pelvis irradiation, although preliminary reports of successful results using newer porous metal technology suggests that this recommendation may need to be revisited.
The presence of pelvic discontinuity and severe bone loss Paprosky type IIC or III may warrant the use of techniques that can provide more stability for the implant in the setting of compromised bone stock. Highly porous metal components In recent years, highly porous metal components have become popular options for both primary and revision arthroplasty procedures. Tantalum implants Trabecular Metal, Zimmer, Inc, Warsaw, Indiana, United States were developed to provide increased porosity and a trabecular bone-like configuration to allow for rapid and extensive bone in growth along with good initial stability in bone.
Some designs incorporate a locking mechnism for the polyethylene insert, whereas others require a cemented polyethylene liner, which allows for placement of the shell in the areas of large defects and compensatory orientation of the cemented liner to re-establish femoral head coverage and hip stability. Modular revision systems that use porous metal augments have been developed.
The ultimate goal of revision acetabular reconstruction should be to obtain stable fixation and restore the hip center[ 5 ]. Previous studies have demonstrated the relationship between polyethylene wear and progressive osteolysis with compromised bone stock[ 9 ]. Liner exchange with highly cross-linked polyethylene has been shown to decrease average wear rates significantly[ 10 ]. To justify isolated liner exchange, the modular metallic shell should be well-fixed and appropriately oriented[ 11 ]. This should be evaluated both pre-operatively and intra-operatively. Once the stability of the acetabular prosthesis is confirmed and liner exchange is contemplated, it is then important to consider the adequacy of the locking mechanism between the liner and the metallic shell. If the locking mechanism is compromised, one may consider cementing a new liner into the fixed metallic shell to prevent micromotion between the two surfaces for primary fixation.
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Paprosky Introduction The aging of the population and the expansion of the number of younger patients with degenerative conditions of the hip have led to an increase in the number of primary total hip arthroplasty THA procedures. The burden of revision surgery is expected to increase in the coming years due to the greater prevalence of patients living with a hip arthroplasty along with overall greater life expectancy. The indications for acetabular revision include symptomatic aseptic loosening, instability, periprosthetic infection, periprosthetic osteolysis, and bearing surface wear. Revision surgery may also be indicated in an asymptomatic patient with progressive osteolysis, severe wear, or bone loss that could compromise a future reconstruction.
Reconstruction options for acetabular revision