Orthopedics

Porous Metal Augments: Big Hopes for Big Holes

Scott M. Sporer, MD, MS; Craig Della Valle, MD

  • Orthopedics. 2010;33(9)
  • Posted September 1, 2010

Abstract

The majority of acetabular defects can be reconstructed with a hemispherical acetabular component alone. However, severe bone loss encountered at the time of revision acetabular surgery can result in compromised initial fixation leading to component micromotion, failure of bone ingrowth, and mechanical failure. Porous acetabular augments offer a potential solution to enhance initial component fixation while maximizing the area for biologic ingrowth. Premanufactured metal augments can be placed independent of the acetabular component. This feature provides surgeons the flexibility to place the hemispherical acetabular component in the optimal orientation at the anatomic hip center. The augment is initially secured to the hemipelvis with multiple screws before it is then secured to the hemispherical acetabular component with the use of polymethylmethacrylate bone cement. The modularity of the augments effectively allows a custom implant to be designed intraoperatively. Porous acetabular augments offer the potential advantages of a nonresorbable material that can be inserted without extensive soft tissue stripping. Additionally, these metal augments do not carry the risk of disease transmission associated with bulk structural allograft and are manufactured in a variety of shapes and sizes. The early clinical and radiographic results of porous acetabular augments are promising, yet there remains a paucity of long-term data.

Durable long-term cementless acetabular component fixation relies on immediate component stability to allow subsequent biologic ingrowth. Fortunately, the majority of acetabular revisions encountered can be reconstructed with a press-fit cementless acetabular component with supplemental acetabular screw fixation. Component survival rates of >90% can be expected using this method of acetabular reconstruction of Paprosky Type I and Type II patients.1 However, severe acetabular bone loss affecting the superior dome, anterior column, or posterior column (Paprosky Type IIIA and Type IIIB) can lead to difficulty in achieving initial stability through the use of a hemispherical acetabular component alone. Various surgical techniques using either bulk structural allograft bone or porous acetabular augments have been described to reconstruct deficient acetabular bone and provide improved acetabular component stability. While structural distal femoral allograft may offer the potential for bone reconstitution, the 10-year clinical results show a >25% failure rate with many of the patients demonstrating significant graft resorption.2

Porous acetabular augmentation is an attractive alternative to a structural allograft. This reconstruction method results in a biologic fixation method without concern for graft resorption. Additionally, the augments can be placed independent of the hemispherical acetabular component to optimize acetabular component position while maximizing the contact with the host bone. Acetabular augments can be placed with minimal soft tissue stripping along the iliac wing, which may also result in decreased operative time and potentially, a decreased risk of infection. Porous acetabular components are currently available from multiple orthopedic manufacturers. While the specific materials and manufacturing processes may be different, all porous augments consist of a highly porous material (titanium or tantalum) with a high coefficient of friction.

Porous acetabular augments are most commonly used to reconstruct a deficient acetabular dome (Paprosky Type IIIA) or a deficient medial wall with segmental bone loss of the anterior or posterior column (Paprosky Type IIIB). Once the location of the desired hip center has been identified, progressive reaming is performed to engage the bone of the anterior and posterior columns. A trial component can then be placed in the appropriate amount of version and abduction. The trial acetabular component will obtain marginal stability until an augment is placed to supplement the stability. A posterosuperior augment is most commonly used in Paprosky Type IIIA defects (Figure 1) while an anterior medial augment is most commonly used in Paprosky Type IIIB defects (Figure 2). The augment can be placed in…

Abstract

The majority of acetabular defects can be reconstructed with a hemispherical acetabular component alone. However, severe bone loss encountered at the time of revision acetabular surgery can result in compromised initial fixation leading to component micromotion, failure of bone ingrowth, and mechanical failure. Porous acetabular augments offer a potential solution to enhance initial component fixation while maximizing the area for biologic ingrowth. Premanufactured metal augments can be placed independent of the acetabular component. This feature provides surgeons the flexibility to place the hemispherical acetabular component in the optimal orientation at the anatomic hip center. The augment is initially secured to the hemipelvis with multiple screws before it is then secured to the hemispherical acetabular component with the use of polymethylmethacrylate bone cement. The modularity of the augments effectively allows a custom implant to be designed intraoperatively. Porous acetabular augments offer the potential advantages of a nonresorbable material that can be inserted without extensive soft tissue stripping. Additionally, these metal augments do not carry the risk of disease transmission associated with bulk structural allograft and are manufactured in a variety of shapes and sizes. The early clinical and radiographic results of porous acetabular augments are promising, yet there remains a paucity of long-term data.

Durable long-term cementless acetabular component fixation relies on immediate component stability to allow subsequent biologic ingrowth. Fortunately, the majority of acetabular revisions encountered can be reconstructed with a press-fit cementless acetabular component with supplemental acetabular screw fixation. Component survival rates of >90% can be expected using this method of acetabular reconstruction of Paprosky Type I and Type II patients.1 However, severe acetabular bone loss affecting the superior dome, anterior column, or posterior column (Paprosky Type IIIA and Type IIIB) can lead to difficulty in achieving initial stability through the use of a hemispherical acetabular component alone. Various surgical techniques using either bulk structural allograft bone or porous acetabular augments have been described to reconstruct deficient acetabular bone and provide improved acetabular component stability. While structural distal femoral allograft may offer the potential for bone reconstitution, the 10-year clinical results show a >25% failure rate with many of the patients demonstrating significant graft resorption.2

Porous acetabular augmentation is an attractive alternative to a structural allograft. This reconstruction method results in a biologic fixation method without concern for graft resorption. Additionally, the augments can be placed independent of the hemispherical acetabular component to optimize acetabular component position while maximizing the contact with the host bone. Acetabular augments can be placed with minimal soft tissue stripping along the iliac wing, which may also result in decreased operative time and potentially, a decreased risk of infection. Porous acetabular components are currently available from multiple orthopedic manufacturers. While the specific materials and manufacturing processes may be different, all porous augments consist of a highly porous material (titanium or tantalum) with a high coefficient of friction.

Surgical Technique

Porous acetabular augments are most commonly used to reconstruct a deficient acetabular dome (Paprosky Type IIIA) or a deficient medial wall with segmental bone loss of the anterior or posterior column (Paprosky Type IIIB). Once the location of the desired hip center has been identified, progressive reaming is performed to engage the bone of the anterior and posterior columns. A trial component can then be placed in the appropriate amount of version and abduction. The trial acetabular component will obtain marginal stability until an augment is placed to supplement the stability. A posterosuperior augment is most commonly used in Paprosky Type IIIA defects (Figure 1) while an anterior medial augment is most commonly used in Paprosky Type IIIB defects (Figure 2). The augment can be placed in any position or orientation to improve the initial stability of the construct, and the bone or the augment can be contoured with a burr to optimize the surface contact area. Polymethylmethacrylate cement is placed directly onto the porous metal revision cup in the areas where it contacts the augment which will unitize the construct once the cement hardens. The acetabular component is then firmly impacted to achieve a press-fit against the host bone and the augment prior to placing multiple screws for adjunctive fixation. As many screws as possible are placed to augment stability. Fresh frozen cancellous bone graft can be packed into any remaining contained defects prior to placement of the cup.

Figure 1A: The severe defect of the acetabular dome Figure 1B: Porous metal augment recreating the superior dome
Figure 1: Radiograph of a 64-year-old woman following resection arthroplasty for a deep periprosthetic joint infection at an outside hospital; the patient sustained multiple dislocations of her articulating spacer. Note the severe defect of the acetabular dome (Paprosky Type IIIA) (A). Postoperative radiograph demonstrating porous metal augment recreating the superior dome which allowed for restoration of the hip center of rotation to its anatomic position (B).

Figure 2A: A Paprosky Type IIIB acetabular defect Figure 2B: Two porous augments were required to reconstruct the anterior column and superior dome
Figure 2: Radiograph of a 46-year-old woman with a Paprosky Type IIIB acetabular defect. Note medial migration and disruption of the anterior column (A). Postoperative radiograph demonstrating restoration of the hip center. Two porous augments were required to reconstruct the anterior column and superior dome (B).

Results

The early results of porous metal augments appear promising in these severe acetabular defects. Sporer and Paprosky3,4 demonstrated no failures due to aseptic loosening in 28 hips with a Paprosky Type IIIA defect or 13 patients with a Type IIIB defect and an associated pelvic discontinuity. Weeden and Schmidt5 also reported no revisions for aseptic loosening when reviewing 43 Paprosky Type IIIA or Type IIIB acetabular revisions at a mean follow-up of 2.8 years. Lakstein et al6 however demonstrated a 8% failure rate at an average 45 months when Trabecular Metal cups (Zimmer, Warsaw, Indiana) without augments were used in patients with <50% host bone contact.

Conclusion

Porous acetabular augments allow the surgeon to customize the acetabular component to address severe acetabular bone loss. While these augments are not necessary in the vast majority of acetabular revision, they offer a biologic alternative to structural allograft bone. However, long-term clinical and radiographic follow-up is required to fully evaluate this surgical technique.

References

  1. Park DK, Della Valle CJ, Quigley L, Moric M, Rosenberg AG, Galante JO. Revision of the acetabular component without cement. A concise follow-up, at twenty to twenty-four years, of a previous report. J Bone Joint Surg Am. 2009; 91(2):350-355.
  2. Sporer SM, O’Rourke M, Chong P, Paprosky WG. The use of structural distal femoral allografts for acetabular reconstruction. Average ten-year follow-up. J Bone Joint Surg Am. 2005; 87(4):760-765.
  3. Sporer SM, Paprosky WG. Acetabular revision using a trabecular metal acetabular component for severe acetabular bone loss associated with a pelvic discontinuity. J Arthroplasty. 2006; 21(6 Suppl 2):87-90.
  4. Sporer SM, Paprosky WG. The use of a trabecular metal acetabular component and trabecular metal augment for severe acetabular defects. J Arthroplasty. 2006; 21(6 Suppl 2):83-86.
  5. Weeden SH, Schmidt RH. The use of tantalum porous metal implants for Paprosky 3A and 3B defects. J Arthroplasty. 2007; 22(6 Suppl 2):151-155.
  6. Lakstein D, Backstein D, Safir O, Kosashvili Y, Gross AE. Trabecular metal cups for acetabular defects with 50% or less host bone contact. Clin Orthop Relat Res. 2009; 467(9):2318-2324.

Authors

Drs Sporer and Della Valle are from the Department of Orthopedic Surgery, Rush University Medical Center, Chicago, Illinois.

Dr Sporer is a consultant for and receives royalties from Smith & Nephew, and is a consultant for and receives institutional support from Zimmer. Dr Della Valle is a consultant for Angiotech, Biomet, Kinamed, and Smith & Nephew and receives research funding and institutional research support from Zimmer.

Presented at Current Concepts in Joint Replacement 2009 Winter Meeting; December 9-12, 2009; Orlando, Florida.

Correspondence should be addressed to: Scott M. Sporer, MD, MS, Department of Orthopedic Surgery, Rush University Medical Center, 25 N Winfield Rd, Ste 505, Winfield, IL 60190 (scottsporer@gmail.com).

doi: 10.3928/01477447-20100722-29

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