This study revealed that iliac extended fixation successfully reconstructed the hip COR, and achieved satisfactory clinical and radiographic outcomes for patients with severe superior acetabular bone defects in revision THA. There were no statistical differences in the major clinical and radiological results between the IIEF and EIEF technique groups, despite the latter group had more severe proximal migration of COR in preoperative radiographies. We did found statistical difference in the orientation of screws fixing the augments between IIEF and EIEF groups, which had potential biomechanical implications.
In this study, the concept of “iliac extended fixation” is proposed to differentiate it from junta-acetabular fixation, which is close to the original acetabulum. According to the Paprosky classification system, the superior aspect of the acetabulum comprises an important part of the bone defect pattern. Biomechanical studies have revealed that the superior bone serves as the major support structure for acetabular cups and an essential component in the process of transmitting load from the hip to the sacroiliac joint[12, 19, 23], and superior and lateral migration of the COR significantly increases joint reaction force, risk of loosening, and decreased range of motion[4, 17, 18, 21, 24, 35, 37]. In knee revision procedures, Haddad et al. proposed the concept of zone fixation in the metaphysis and diaphysis, while preserving the knee joint line level [12, 22]. Similarly, in our practice of iliac extended fixation technique, viable bone is pursued within the whole semi-pelvis to achieve good primary stability and further biological fixation.
Iliac extended fixation is defined as extending beyond 2 cm superior to the anatomic acetabular rim, based on anatomical and biomechanical rational[1, 4, 17, 18, 21, 24, 35, 37]. Augments used at juxta-acetabular region can be considered a rim restoration procedure, while in iliac extended fixation, a superior or superior posterior/anterior supporting point is created by extending porous metal across the worst quality and quantity of bone potentially remote to acetabular rim. In comparison with juxta-acetabular fixation, iliac extended fixation requires more surgical exposure and is more difficult to obtain adequate primary mechanical stability. Our data revealed that bone ingrowth fixation was identified between the bone-cup and bone-augment interface in all patients, indicating that with iliac extended fixation achieved sufficient initial stability and subsequent biologic osteointegration without compromising COR and cup orientation (Table 3–5).
The iliac extended fixation technique is aimed to reconstruct severe cranial acetabular defects while reconstructing the COR and cup orientation as optimal as we can. In the present study, the postoperative COR were significantly lower compared to the preoperative COR, more close to the anatomic COR (Table 3). The hip biomechanics were therefore improved, which is consistent with previous reports . Restoration of the COR helps decrease leg length discrepancy and avoid increasing the lever arm, which acts on the head/neck taper caused by increased neck length . Moreover, superior placement of the shell may increase the joint reaction force, in addition to the risk of impingement, loosening and hip instability .
The iliac extended fixation technique can be further classified into two groups of EIEF and IIEF. In this cohort, IIEF contained a single superior slope augment or stacked slope augments mounted into the cavitary bone defects, usually below the level of sacroiliac joint. As the EIEF featured no cavitary bone to prevent the augments from migrating proximally, buttress augments were utilized to extend the construct to the level of sacroiliac joint. There were several studies reporting the use of porous metal augments in revision THA with promising short term outcomes [3, 8, 13, 32]. Despite this, very few studies have reported the classifications of augment reconstructions in the superior acetabular area. Derek et al reported 5–12 year follow-up of 85 revision THA cases with porous metal augments and classified the use of slope augments in the superior aspect into the three types, namely “flying buttress”, “dome” and “footing”. This classification is not contradictory to the concept of iliac extended fixation, as the essence of extended fixation is the bridging of bone defects via augments to establish a supportive superior point; any augment configuration can be used as long as mechanical stability and anatomical COR are achieved. To the best of our knowledge, the dome and footing techniques could fall into the IIEF group, and the flying buttress technique could be the subtype of EIEF if it meets the definition of being more than 2 cm superior to the anatomic acetabular rim.
Our data showed that the EIEF group had similar postoperative vertical and horizontal location of COR compared with the IIEF group, in spite of the higher COR in preoperative radiographies. Spanyer et al previously reported using a buttress augment for successful repairing the anterior column through the direct anterior approach , while Ballester et al reported the application of buttress augments in treatment of severe acetabular bone loss . The restoration of COR using EIEF technique is similar to those of buttress augment reported in the literature.
There are several biomechanical, surgical and postoperative rehabilitation implications of this sub-classification. Our data revealed that the screw orientation was significantly more horizontal in the EIEF group than the IIEF group when measured in reference of the inter-teardrop line (4.6° vs. 41.9°). This difference in screw orientation is most likely a result of different biomechanics of the two groups. The EIEF features high shear force in the augment-bone interface, since no bony wall is available to resist the joint reaction force. This then surgically requires horizontal buttress screws to achieve stable fixation of metal augments with high friction coefficient to counteract the shear stress, and adequate intraoperative exposure of the superior aspect of acetabulum usually to the level of sacroiliac joint. Differently, superior migration of metal augments is prevented by bony walls in IIEF, which then features mainly compressive forces at the augment-bone interface. Surgically, IIEF requires oblique screws perpendicular to the metal-bone interface to counteract the tangential shear force while allowing potential dynamic compression along the screws. In our practice, rehabilitation for patients with EIEF was generally more conservative compared with that for patients with IIEF, who could be allowed with earlier weight bearing according to the intraoperative evaluation of the reconstruct stability.
The EIEF group had higher rate of osteointegration in zone 1A compared with the IIEF group (91.7% vs 50.0%, Table. 5). This might be caused by better bone-augment contact in zone 1A of the EIEF group, and indicated that the EIEF technique successfully bridged the bony fixation of acetabular cup to the ilium. There is higher rate of radiolucent lines in zone 1A in the IIEF group than in the EIEF group, although not statistically significant (p = 0.075, Table. 5) It is still unknown whether the radiolucent lines in zone 1A will cause possible loosening of the cementless construct, which necessitates longer term of follow up.
This study had several limitations. First, due to the short follow-up duration and small sample size, survivorship analysis and comparisons with other reconstruction methods were not conducted. The restoration of hip COR and good bone ingrowth in iliac extended fixation may contribute to improved hip function and long-term implant survivorship. Further study comparing the iliac extended fixation technique with other techniques should be compared. Second, the cement bond between the stacked metal components must be evaluated with long term follow up or in vitro fatigue tests to exclude micromotion and subsequent generation of abrasion. Third, it requires further biomechanical analyses to assess any potential differences in mechanical properties between juxta-articular within 2 cm of acetabulum and iliac extended reconstructions beyond 2 cm of acetabulum, as well as between EIEF and IIEF reconstructions.
The findings of this study indicate that EIEF and IIEF with porous metal augments and cementless cups are effective in reconstructing severe superior acetabular bone defects, with promising short-term clinical and radiographic outcome. The EIEF technique achieved satisfactory clinical and radiological outcomes similar to the IIEF technique in spite of higher preoperative COR, and required more horizontal intraoperative screw fixation for the augments.