Managing bone defects using large acetabular cups and highly porous augments
Revision total hip arthroplasty (rTHA) is a difficult surgical procedure. The key challenge of this procedure is the potentially significant bone loss and the often encountered poor bone quality. Although much advancement in surgical techniques, technologies, and materials has been made in recent years and has helped improve the treatment of complex acetabular defects, several topics remain controversial, and a systematic incorporation of the technical advancement into treatment paradigms is still needed [1]. In this article, George A Macheras, Head of the Orthopedic Department in the Henry Dunant Medical Center, Athens, Greece, shares his experience and his strategies in managing acetabular bone defects. Examples of how cases of severe bone loss were managed using advanced techniques are also presented.
George A Macheras
Henry Dunant Medical Center, Athens, Greece
The Paprosky Classification
To perform a successful rTHA, it is crucial to accurately assess the severity and the pattern of bone loss so as to predict the ability of the remaining host bone to support the implants and maximize the amount of initial stability until bone ingrowth can occur [2]. The goal of an optimal solution is to restore hip biomechanics and achieve rigid fixation for long-term results.
As has been addressed in Part 1, among the various classification systems, the Paprosky Classification has demonstrated good validity and moderate reliability. When a surgeon is trained in this system, it can provide good reliability [3]. It is also the most clinically oriented because it assesses both the amount of acetabular bone loss and the ability of the remaining host bone to support the initial stability of the acetabular components [4]. In case of severe bone loss, the Paprosky Classification is a particularly useful treatment guide [1, 5].
Four radiographic features are important in determining the Paprosky types, ie, the teardrop, ischium, Köhler’s line, and superior migration (Figure 1). These four features indicate the integrity of the medial wall (teardrop), posterior column (ischium), anterior column (Köhler’s line), and superior dome (superior migration of the hip center) [6]. The criteria for the Paprosky Classification are summarized in Table 1 [4].
Ask the expert
We asked George Macheras how accurate a preoperative assessment of bone loss is in general. He told us that, intraoperatively, one should always be ready to encounter greater bone loss than predicted by the preoperative assessment. A meticulous debridement that rids the acetabulum of necrotic bone and scar tissue is pivotal, which should be followed by reaming the defects until bleeding healthy bone. Only then can the true bone loss be evaluated. For this reason, one should always have more than one reconstruction option available at the time of surgery.Some general principles in treating acetabular bone loss are presented below.
Modern advancement to enhance biological fixation
As clinical evidence showed superior outcome of using biological compared to nonbiological fixation, cementless fixation has become the preferred method for acetabular revisions [2]. In biological fixation, initial mechanical stability is required to provide an environment that allows bony ingrowth, and intimate contact between the implant and viable bone is required to promote osseointegration [2]. Modern implant surfaces for contacting host bones such as highly porous tantalum or porous titanium allow faster bone ingrowth into the implant surface and are available today and in use in everyday practice. These materials have interconnecting pores of regular shape and size and high porosity of 75–80% in volume, which is higher than that of fiber metal coatings (40–50%) and sintered beads (30–35%) [7]. The result is a biomechanical property similar to that of cancellous bone allowing a faster and greater volume of bony ingrowth and greater increase in interface strength [7].
Highly porous trabecular metals have been used as augments that come in different shapes and sizes (see section on highly porous metal augments). These augments fill the bone defects in the acetabular wall, support the metal shell of the acetabular prostheses, allow the restoration of the center of rotation and hip biomechanics, and help maximize the host bone contact to the prosthesis, thus promoting osseointegration. Satisfactory mid- and long-term results from the use of those materials have been reported [8–10].
As already mentioned, initial stability is of paramount importance and therefore screw fixation in addition to the press-fit fixation is required in most cases.
General principles in revision surgery:
- Clean soft tissue meticulously from the acetabular wall
- Recognize the bone defects
- Make sure there is no pelvic discontinuity
- Ream gently to create a hemisphere
- Protect the anterior and posterior walls
- Ream to bleeding bone
- Fill the bone defects with reamed bone or allograft
- Use highly porous cups
- Look for rim fixation
- Maximize bone contact (aim for 50% host bone coverage)
- Enhance fixation using multiple screws
Jumbo cups
Large diameter cups are usually used in revision cases with bone defects [6, 11]. In some cases, the so-called jumbo cups are used. Various definitions have been used for the term “jumbo cup”, with the most prevailing definition being a shell diameter of > 66 mm for men and > 62 mm for women [11, 12].
Jumbo cups have several advantages: The surgical technique is simpler, more straightforward, and more reproducible. Due to the larger surface area of the cup, it provides a larger contact area with the host bone, and thus reduces (or even eliminates) the need for bulk allograft or metal augments [6, 11, 13]. The general principle of primary initial stability is achieved when jumbo cups are used—aside from its large surface area, one can take advantage of the remaining vital bone that usually exists in the pubic and ischial area and achieve even more contact between the cup and vital bone and restore the center of rotation at the same time. Also, to achieve maximum initial stability, as many screws as possible should be used to the vital pelvic bone, preferably over the sciatic notch where strong bone exists [14].
Concerns have been raised that in using jumbo cups, the elliptical type II and III defects are converted into hemispherical sockets, which may lead to a higher hip center—a biomechanically less favorable situation prone to higher risk of dislocation, leg length discrepancy, and loosening (for AAOS classification, see Table 1 above or Part 1) [1, 6]. A study by Nwankwo and Ries [13] showed that jumbo cups elevated the center of rotation by about 1 cm on average. This is within an acceptable range to some [2], although George Macheras advises us to stay within the 5-mm limit to avoid a significant change in hip biomechanics.
Recent evidence indicated favorable long-term outcomes for these implants [11, 15].
Highly porous metal augments
Metal augments come in different shapes and sizes (Figure 2) and their purpose is to cover bone defects in the acetabulum, support the acetabular prosthesis, and offer immediate stability. They are made from the same material as the acetabular prosthesis, ie, trabecular titanium or tantalum.
Managing type I defects is relatively straightforward
The hallmark of Paprosky type I acetabular bone loss is the intact rim with minimal bone loss and retention of the hemispherical shape of the acetabulum [4]. To manage this type of defects, first debride meticulously to remove the scar tissue and pseudomembrane and then ream appropriately to bleeding bone. A regular hemispherical cementless shell with or without screws can now be inserted, taking care that initial stability is provided. Shells with modern surface treatments such as highly porous titanium or tantalum are advocated [16].
General principles in Paprosky type II management
For Paprosky type II defects, see Table 1 and Part 1. The common characteristics of type II subtypes are the distortion of the acetabular substrate but intact anterior and posterior walls, as well as the superior migration of the cup less than 3 cm [6].
The first step of the operation is to identify the true bone defects and recognize the remaining vital bone [2]. Therefore, meticulous debridement of the scar tissue and pseudomembranes and careful reaming of the acetabular wall to bleeding bone are necessary. Any cavitary defect should be recognized; scar tissue and the pseudomembranes should be completely removed and filled with either the reamed material or allograft.
In general, type II defects can be treated with regular, appropriately sized uncemented hemispherical components with modern, highly porous surfaces (eg, trabecular tantalum or trabecular titanium), as they have better properties that provide mechanical stability and biological fixation. Jumbo cups may be needed in some cases. If necessary, defects can be filled with cancellous allograft.
Type IIA defects
In Paprosky type IIA, both anterior and posterior walls are intact and the superior cup migration is less than 2 cm; the management principle laid out above should be sufficient for treating this type of defects [4].
Type IIB defects
In type IIB defects, the anterior and posterior walls are intact and supportive, but there is bone loss in the dome of the acetabulum [4].
Most reconstructions can be successfully performed with hemispherical uncemented cups with modern, highly porous contact surface. Bone defects less than 30% can be ignored [4] or filled with reamed material or allograft. If the superior defects are larger than 30%, appropriately sized metal augments could be used (see Figure 3). In case of severe superior bone defects, extra-large cups or jumbo cups may be needed for increased implant-host bone contact. It has been shown that a complete coverage of the acetabular component with host bone is not necessary; as a matter of fact, even a 35–40% coverage with vital bone is acceptable [1]. Nevertheless, as the failure mode for most acetabular components involves the loss of inferior fixation (“up and out”), one should take advantage of the availability of good vital bone in the pubic and ischial parts of the acetabulum and use as many screws to the pelvis as possible for extra stability (Figure 3). This may not be easy and is technically challenging, even if that is the ideal way of fixation.
There are two ways of using the cup-augment construct:
- Fixing the appropriately sized augment to the pelvis and then the cup
- Fixing the cup to the pelvis and then fixing the augment.
The downside of the first option is that it is difficult to drive the screws for cup fixation through the augment. This is possible but only with a diamond drill.
Type IIC defects
In type IIC defects, the medial wall is absent and the Köhler’s line is disrupted to the point of teardrop obliteration, causing the medial migration of the cup [4]. As anterior and posterior walls are intact and the dome is not severely damaged, a hemispherical component could achieve stable fixation. Most type IIC defects can be treated in a similar way as the type IIB defects, except that particulate bone graft or metal augment of appropriate size and shape will be needed to cover the defect in the medial wall. The stability of the implant is achieved through the intact acetabular rim plus the use of as many additional screws to the vital bone as possible. The goal is to restore the hip center of rotation and hip biomechanics by restoring the anatomy and function of the abductors.
General principles in Paprosky type III management
Type III defects are associated with severe bone loss and unsupportive host bone. Since the cup migration is more than 3 cm, a hemispherical cup alone is less likely to be successful. Nevertheless, many type IIIA cases can be successfully treated using the same principles as Paprosky type II defects. In some cases of large defects in the superior dome, the cups can be placed slightly higher [1, 6]. Similarly, if a jumbo cup is used and stability can be achieved in a higher position, a higher hip center is an acceptable solution. In general, a higher hip center of up to 5 mm is acceptable.
Due to the larger defects in type IIIA, structural support with metal augment, femoral allograft, cage, and/or a cup-cage construct is needed. Custom implants with highly porous contact surface are helpful in managing large defects and have shown good outcomes [1, 17].
When using augments, the augment is usually fixed with the trial cup in place. Cancellous allograft is then filled in and the final cup is inserted and pushed against the fixed augments. In some cases, more than one augment could be used—the so called “Lego technique”. However, this technique may vary depending on the bone defect and the bone quality. In some cases, the final cup is first inserted by press fit and secured with screws to gain initial stability before the augments are placed. With this method, augments used in wedge orientation can be impacted against the final acetabular component. In both alternatives, the augments should be secured to the acetabular component with screws and a thin layer of bone cement is used between the augment and the cup in order to avoid possible wear products from the friction between the two implants (Figure 4) [1].
Type IIIA defects
This type of defect presents with bone loss from "10 am to 2 pm" and is associated with superior or superolateral cup migration of more than 3 cm. Köhler’s line and teardrop are intact and there is minimal ischial bone lysis [4]. The defect could be superior (Figure 5), posterosuperior (Figure 6), or extensive segmental (Figure 7).
Nonbiological fixation such as a combination of cancellous allograft and cage can be used—for example, when the anterior column is not supportive and the Köhler’s line cannot be restored by using only metal augments; in this case, a combination with a cage is more appropriate. In case of a nonsupportive posterior column, augment or structural allograft is needed to provide rigid fixation and a cage supplement will be needed.
Cup-cage construct for handling type IIIB defects
In patients with massive acetabular bone loss or pelvic discontinuity, a cup-cage construct can be an effective tool [1]. Figures 13–16 demonstrate cases where the cup-cage technique was used in type IIIB cases.
In managing type IIIB defects, after a thorough soft-tissue debridement, reaming of the remaining bone until bleeding bone is performed. Afterwards, it is important to check the real bone defect and reestimate the bone loss; one should pay special attention to the possibility of the existence of pelvic discontinuity. The treatment strategy should include a decision on whether multiple augments with jumbo cups, cup-cage, or custom implants are warranted.
In case of a supportive anterior column, a biological fixation of restoring the Köhler’s line with augments in the footing position and bone allografts can be performed (Figure 12).
Type IIIB defects
In Paprosky IIIB defects the bone loss and disruption is severe, migration is more than 3 cm, Köhler’s line and teardrop are disrupted, and there is severe ischial bone lysis [4]. Figure 11 depicts how this type of defects may be managed by using the appropriately sized metal augment.
Intraoperative assessment determines the actual treatment method
Figure 10 demonstrates how true defects may only be discovered intraoperatively. The loose implant was first removed and the defect in the acetabular wall was then meticulously debrided. This allowed the identification of the true defect in the medial wall in addition to the defect in the superolateral aspect. The superior defect was covered by augments and the medial wall was covered by cancellous allograft before a cup was inserted.
Example of type IIIA cases
Figure 8 is from a case with a significant defect in the superior aspect and Figure 9 is from a case with major superolateral bone defect. These two cases demonstrate how multiple screws were used to fix both the augment and the cup. Macheras tells us, “occasionally, a surgeon can choose to drive a screw from the cup and through the augment to the host bone—preferably over the sciatic notch where good quality bone still exists; however, this can be technically challenging”. Special instruments, such as a diamond drill, are needed in this case.
Figure 9 shows another type IIIA case with major superolateral bone defect, where a “up and out” cup is clearly seen. In both cases, a large cementless trabecular metal cup was used, although the cup sizes did not quite reach the definition of jumbo cups.
Conclusion
Bone defects after failure of the acetabular prosthesis are common. For these cases, an accurate bone loss estimation is of paramount importance and the management can be very demanding. The main aim is the restoration of bone stock, hip biomechanics, and hip function. In our experience, intraoperative findings can be worse than the preoperative estimates, therefore surgeons must have a plan B. An rTHA is a complex and difficult surgery that involves many surgical tips and tricks; such a surgery is best handled by an experienced surgeon.
Contributing experts
This series of articles was created with the support of the following specialists (in alphabetical order):
Theofilos Karachalios
University General Hospital of Larissa at the University of Thessaly, Larissa, Greece
Thomas Kostakos
Henry Dunant Medical Center, Athens, Greece
George A Macheras
Henry Dunant Medical Center, Athens, Greece
The authors thank Maio Chen, medical writer at AO Innovation Translation Center, Switzerland, for contributing to the writing and editing of the articles.
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