|Year : 2018 | Volume
| Issue : 4 | Page : 193-197
A biomechanical approach to advances in sacropelvic reconstruction
Peter S Rose1, Michael J Yaszemski2, Franklin H Sim1
1 Department of Orthopedic Surgery, Mayo Clinic, Rochester, Minnesota, USA
2 Department of Orthopedic Surgery and Biomedical Engineering, Mayo Clinic, Rochester, Minnesota, USA
|Date of Web Publication||9-Nov-2018|
Prof. Franklin H Sim
Department of Orthopedic Surgery, Mayo Clinic, Rochester, Minnesota
Source of Support: None, Conflict of Interest: None
The sacrum is the only mechanical connection between the spine and pelvis/lower extremities. In the setting of a primary, or in select patients a locally advanced recurrent malignancy, curative treatment requires en-bloc sacrectomy. In addition to the surgery in this area being challenging due to the complexity of the pelvic anatomy, adjacent visceral and vascular structures; spinal-pelvic continuity is often lost. Historically following sacral resection patients were left “un-reconstructed” and the spinal column floated between the remaining pelvis, forming a soft-tissue sling which can become painful and lead to a poor patient outcome. Our institution has pioneered a means to reconstruct these defects following high sacral resection in order to restore continuity between the spine, pelvis and femur which has been shown to improve patient outcomes. The purpose of this article is to describe a biomechanical approach to sacral reconstruction.
Keywords: Biomechanics, orthopaedic and plastic, reconstruction, sacral resection
|How to cite this article:|
Rose PS, Yaszemski MJ, Sim FH. A biomechanical approach to advances in sacropelvic reconstruction. Hamdan Med J 2018;11:193-7
| Introduction|| |
En bloc resection is considered a mainstay of treatment for primary tumours of the sacrum.
In the setting of a high sacral resection, spinopelvic continuity is compromised making reconstruction necessary. Reconstruction is difficult and requires a team of orthopaedic and plastic and reconstructive surgeons to restore the segmental bony defect and the complex biomechanics of the sacrum. The purpose of this review is to describe the challenges reconstructive surgeons face following sacral resection, and the authors’ suggestions to address those challenges.
| Sacral Biomechanics|| |
As the only mechanical connection between the spine, pelvis and lower extremities, the sacroiliac (SI) joints are highly constrained. The wedge shape of the sacrum between the iliac wings, similar to a ‘keystone’ provides inherent stability to prevent caudal migration. In addition, the SI joint has an irregular and broad surface which interlocks the sacrum to the ilium. These joints must resist not only compression but also rotation, allowing for translation of only 0.7 mm and 2° of motion., Similarly, the broad surface of the SI joint allows the joint to not only resist compressive forces in addition to shear forces. In addition to the bony articulations, the intraosseous sacroiliac ligaments, the sacrotuberous ligaments, the sacrospinous ligaments and the iliolumbar ligaments provide further stability.
Following a high transverse sacral resection, the biomechanical integrity of the SI joint is compromised by up to 50%; however, this amount of weakening often does not prevent safe weight-bearing., These early studies were expanded on with a cadaveric model by Hugate et al. which showed a transverse partial sacrectomy performed cranial to the S1 nerve root [Figure 1]a placed the residual sacrum at substantial risk of a midline sagittal plane fracture during the activities of daily living. Due to this study, reconstruction was recommended for these patients. However, if the resection was below the S1 sacral segment [Figure 1]b, it was felt the sacrum would be able to withstand postoperative mobilisation. This was expanded on by Yu et al. showing a similar outcome when a resection occurred at the S1 level. With resection at the S1 level, the authors noted rotational instability, and further cephalad resection through half of the S1 body lead to compressive instability. The results of this series lead the authors to recommend reconstruction when the transverse osteotomy was at the S1-S2 vestigial disk 1eve1. These studies have formed the standard of practice at our institution, and we advocate for reconstruction if there resection of one of the SI joints or if there is a transverse osteotomy above the S1-S2 vestigial disk. This allows for the restoration of spinopelvic continuity, improving functional outcomes compared to patients without reconstruction.,,
|Figure 1: Through biomechanical testing, saciectomies performed cranial to the S1 neural foramen (a) were found to be unable to withstand postoperative mobilisation compared to osteotomies performed caudal to the S1 level (b). As a result of these findings, we recommend reconstruction if the osteotomy is performed cranial to the S1 foramen|
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| Spinopelvic Reconstruction|| |
The goal of spinopelvic reconstruction is to restore spinal balance. To accomplish this pedicle and iliac screw fixation with posterior rods and anterior support in the form of fibular struts (namely vascularised free fibula grafts) has become our standard technique. Previous reconstructive techniques utilising spinal instrumentation was associated with a high failure rate, likely due to the use of posterior-only techniques without anterior column support.
Due to this, we began to use, newer combined anterior and posterior reconstruction techniques have reduced failure rates.,,,,
Iliac fixation is based on the two pathways of abundant cancellous bone as follows: the upper (posterior superior iliac spine to the iliac crest) and lower segments (posterior superior spine to the anterior inferior iliac spine) provide excellent purchase for screws. Screw placement in both these tracks allows for significant resistance to compressive and torsional forces especially with dual lower segment fixation. In addition to improved iliac fixation techniques, changes have occurred in posterior rod instrumentation techniques, namely the four-rod reconstruction technique with cross-linking provides significantly greater stability in flexion and extension and rotation following sacrectomy. In addition to a four-rod reconstruction with cross-linking, our preferred reconstruction technique includes an anterior column reconstruction with a strut graft (which are often preferably vascularised free fibulas), forming a ‘cathedral-shaped’ reconstruction of the anterior spinal column at the sacral level.,
Indications for reconstruction
At our institution, reconstruction is performed when the SI joint has been disrupted, based on the Mayo Spino-Sacro-Pelvic Tumour Classification [Figure 2], following either a total sacrectomy (Mayo Type IA), partial sacrectomy above the S1 foramen (Mayo Type IB), unilateral hemisacrectomy (Mayo Type II), hemipelvectomy and partial sacrectomy (Mayo Type III) or total sacrectomy and hemipelvectomy (Mayo Type IV). Reconstruction is not necessary for a partial sacrectomy which does not disrupt the SI joints (Mayo Type IC).
|Figure 2: The Mayo Sacral Tumour Classification is to assist in determining if reconstruction is necessary following a sacral tumour resection. We recommend the reconstruction for cases of Mayo Type IA (total sacrectomy, a), Type IB (subtotal sacrectomy above the S1 foramen, b), Type II (hemisacrectomy involving the sacroiliac joint, d), Type III (hemipelvectomy and hemisacrectomy, e) and Type IV (total sacrectomy and hemipelvectomy, f). Since the SI joints are not disrupted in a Type IC (subtotal sacrectomy below the S1 foramen, c), a reconstruction is not typically performed|
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Staging of a sacrectomies which require a reconstruction, has been shown to reduce patient morbidity and is currently our preferred technique. The results of this series showed reduced complications, cost, ICU stay and transfusion requirements when patients were staged compared to completing these procedures in a single setting. We typically perform this procedure in two to three stages on two or three different surgical days. The anterior procedure, which we perform on day 1, includes the harvesting and banking of a vertical rectus abdominis myocutaneous (VRAM) flap, mobilisation of the viscera and vascular structures and osteotomies in the sacrum, spine and pelvis based on the tumour location. The second stage is the posterior approach and dissection, completion of osteotomies and tumour delivery and provisional VRAM flap inset followed by wound closure. Based on the size of the resection the patient then gets a spinopelvic computed tomography (CT) scan for planning for the reconstruction, or the reconstruction is completed during the second stage. The Stage 3 reconstruction consists of pedicle screws placed in a bicortical fashion in the two or three most distal vertebrae, and one or two iliac screws on each side. The anterior column reconstruction consists of creating docking sites for the two fibulae with a burr in the most caudal portion of the lowest remaining vertebra and in the ilia. The locations of the ilia docking sites are based on the postresection CT scan and are typically located in the supraacetabular region, the ischial ramus or in the ischial tuberosity.
The ideal position of the docking sites in the ilium is at the intersection of the iliopectineal line, and a straight line which intersects the centre of the hip joint and the most caudal remaining vertebra [Figure 3]a.
|Figure 3: During the anterior exposure of the tumour, docking sites for the strut grafts are made on the iliopectineal line at the crossing point from the centre of the planned distal vertebrae and the centre of the hip joint. Bicortical pedicle screws are placed in the remaining lumbar vertebrae, once the tumour has been resected. Similarly, docking sites are made in the central portion of the most distal vertebrae (a). The fibular strut grafts are then impacted into the place of the ilium and then inserted into the distal vertebrae in a ‘cathedral’ fashion (b). The construct is then compressed, and the nuts of the pedicle screws are tightened (c)|
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Fibular allografts or our current practice is to use vascularised fibular grafts are fashioned to be inserted into the docking sites, creating a ‘cathedral’ appearance [Figure 3]b. The fibulae are docked into the most distal remaining vertebra, and a bilateral four-rod posterior instrumentation reconstruction with cross-links is performed, while compressing the instrumentation construct across the docking site [Figure 3]c.
The soft-tissue reconstruction is then performed using a VRAM flap which is brought through the centre of the cathedral [Figure 4]. A dermal replacement is then used to reconstruct the abdominal wall to prevent bowel and visceral herniation. It is important to leave a space in the caudal portion of the dermal replacement to allow for the VRAM pedicle. In addition, a transfer of the obturator nerve can be performed to provide a reconstruction of the pudendal nerve and potentially restore bladder function. The VRAM flap is inset using a closure of the fascial, subdermal and skin levels, and the patient is kept on a specialised pressure equalising bed mattress to allow for optimal flap healing.
|Figure 4: Following reconstruction, a biologic dermal matrix is used to reconstruct the posterior abdominal wall. It is placed on the remaining spinal column and pelvis, with care taken to have an opening at the caudal portion to all passage of the pedicle of the rectus flap|
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A unilateral cathedral technique can be performed where only one SI joint is resected to recreate the biomechanics of the SI joint [Figure 5]. The technique is similar to the bilateral procedure, with a compressive pedicle and iliac screw reconstruction, augmented with a vascularised fibular graft.
|Figure 5: Example of a unilateral cathedral reconstruction using a fibular strut graft and pedicle and iliac screw fixation. The docking sites for the graft are created in a similar fashion, and compression is placed across the graft|
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External hemipelvectomy and sacrectomies are rare, with few options to restore spinopelvic continuity. Spinopelvic continuity is maintained in the setting of an external hemipelvectomy combined partial sacrectomy (Mayo Type III); however, this is not enough to support ambulation and sitting. Although there is no biomechanical data to support this, we advocate for reconstruction when the spinal disc is entered and utilise a lumbar pedicle and iliac screw fixation. Along with iliac fixation, the number of remaining vertebrae instrumented is based on the extent of the tissues removed during tumour resection, patient weight and bone quality. Two to four vertebral levels are instrumented utilising posterior rods and cross-links [Figure 6]a. The construct is compressed, and the caudal iliac screw is connected to the ipsilateral sacral pedicle screw and the contralateral lumbar pedicle screws. In addition, the structural allograft bone is placed in each disk included in the instrumented levels of the arthrodesis [Figure 6]b; however, we currently use a vascularised fibular graft to enhance arthrodesis especially in the setting of previous radiotherapy.
|Figure 6: Example of a complex reconstruction following a Mayo Type III resection. The patient developed severe pain after initially being unreconstructed. The patient was then reconstructed using pedicle and iliac screw fixation with cross-bar linking of the remaining sacrum and pelvis (a). Following the procedure, there was solid arthrodesis, with a substantial reduction in pain (b)|
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In the setting of a total sacrectomy and hemipelvectomy [Mayo Type IV, [Figure 7]a, the amputated proximal femur is used for bone graft as long as there is no tumour located in this region. Similar to a Type III reconstruction, the remaining two to four lumbar vertebrae are instrumented with bilateral bicortical pedicle screws, along with dual or triple iliac screw fixation. An intertrochanteric and subtrochanteric osteotomy through the proximal femoral autograft is performed to restore the anatomic distance between the spine and pelvis [Figure 7]b. The angle of the osteotomy is chosen to allow the amputated femoral autograft to align with the long axis of the patient's body and to minimise either stretching or kinking of the vessels and nerves between the remaining lumbar spine and ilium. A docking site is created in the remaining ilium between the cancellous bone of the amputated femur's intertrochanteric osteotomy to the remaining ilium. This also allows for fixation of the subtrochanteric osteotomy site of the amputated femur into the most caudal remaining vertebra [Figure 7]c. This reconstructive construct is held in place with a compression screw [Figure 7]d and using a screw-rod instrumentation between the lumbar spine and the remaining ilium [Figure 7]e.
|Figure 7: To reconstruct a Mayo Type IV resection (a) the amputated proximal femur is used to restore the height between the spine and pelvis. An osteotomy is performed through the subtrochanteric femur (b), and the proximal femur is ‘flipped;’ hence, the proximal femur's osteotomy rests against the remaining ilium (c). The femur is then compressed to the ilium (d) with multiple pedicle and iliac screws and well as posterior rods (e)|
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| Conclusion|| |
Restoration of spinopelvic continuity improves patient outcome, and the use of the cathedral technique developed at our institution has resulted in an 89% of success rate. Currently, our preferred reconstructive technique is to utilise dual iliac screw fixation, dual posterior rod constructs with cross-linking and anterior column support with a free vascularised fibular graft following sacral resection. Biomechanically, instrumentation failure can be reduced when the anterior column is supported. Although it increases the surgical time, we feel the use of vascularised fibular autografts is a reasonable strategy to improve the reconstruction construct's stability and healing potential.
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Conflicts of interest
There are no conflicts of interest.
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[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7]