Drs. White, Matcuk, Gottsegen, Ahlmann, and Patel are at USC University Hospital, Los Angeles, CA. Drs.Learch, Menendez, and Allison are at Cedars-Sinai Medical Center, Los Angeles, CA; Dr. Szymanowski is at Saddleback Memorial Medical Center, San Clemente, CA; and Dr. Moin is at Pueblo Radiology of Ventura,Ventura, CA.
This article was previously presented as an educational exhibit at RSNA 2008.
neoplastic disease can be metastatic or primary. Most metastatic
lesions originate from prostate, thyroid, breast, lung, and renal
primary malignancies.1 Common locations of osseous metastatic
disease include the spine, ribs, pelvis, and femur (Figure 1). Primary
bone malignancies, most of which are sarcomas, are less common. Within
the United States, approximately 2600 cases are identified annually.2,3 Of these, 5% involve the pelvic girdle.1
Common primary neoplasms affecting the pelvic girdle include Ewing’s
sarcoma during childhood, osteosarcoma during adolescence, and
chondrosarcoma during adulthood.4,5
with a malignancy’s pathologic variant, anatomic location, and the
patient’s physical status. Focal lesions may be treated with
radiotherapy, surgery, or neo-adjuvant therapy consisting of
preoperative chemotherapy or radiation. More extensive lesions may
undergo significant surgical resection and reconstruction to alleviate
pain or maintain the patient’s functionality. The intent of the
resection may be curative or palliative. Oncologic tenets may
necessitate a surgical resection that complicates reconstructive effects
to obtain a successful limb-salvage procedure.
The purpose of
this article is to provide an understanding of the indications,
treatment methods, and complications of saddle endoprostheses, which are
used for pelvic reconstruction after tumor resection. Topics
highlighted include pelvic resection classification, surgical resection
techniques, pelvic reconstruction saddle prosthesis types, and
postoperative imaging, including signs of complications.
Classification of pelvic resections
In 1978, Enneking and Dunham6
classified pelvic resection margins into 3 main types (Figure 2). Type I
resections are confined to the ilium (Figure 3), type II resections are
confined to the periacetabulum, and type III are confined to the pubis
Of these, types I and III do not require acetabular
resection, thereby maintaining pelvic stability and forgoing the need
for prosthesis placement. Type II resection provides the option of
limb-sparing surgery or hind-quarter amputation (external
hemipelvectomy). Crossover of pelvic resection types is common and
resections are classified as such. For example, a resection involving
both the acetabulum and the iliac wing is referred to as a type I/II
pelvic resection (Figure 5). Lesions also can cross zones and still be
treated with a single resection type. This is usually the case with
large metastatic lesions, in which the objective is palliation.
Periacetabular resection techniques
acetabulum is a difficult region in which to operate due to the complex
anatomy and the proximity of neurovascular structures.7
Unlike in the femur, a lesion involving the acetabulum cannot be readily
stabilized by intramedullary fixation or a composite prosthesis. Due to
this and other surgical considerations, the external hemipelvectomy
remained the standard for surgical therapy for large acetabular tumors
until the 1970s.7
hemipelvectomy comprises internal and external subtypes. The external,
or classic, hemipelvectomy was the standard surgical procedure until the
1970s. It consists of resectioning the hemipelvis and the ipsilateral
extremity. While this extensive procedure may cure a primary neoplasm,
the resulting functional deficit is profound. In addition, the procedure
has been associated with high infection rates.8
Improvements in chemotherapy led to limb preservation efforts, resulting in the development of the internal hemipelvectomy.9
This limb-sparing procedure consists of complete or partial resection
of the hemipelvis, with preservation of the ipsilateral limb. Although
the patient maintains a viable limb, functionality is limited due to
significant limb-length discrepancy, flail hip, and gait instability. In
addition, the rehabilitation course can be prolonged. As with external
hemipelvectomy, high rates of infection are associated with this
limb-sparing procedure is iliofemoral or ischiofemoral arthrodesis. The
general premise of arthrodesis is surgically inducing joint ossification
between two bones. This procedure leads to significant limb-length
discrepancy, though patients are sometimes able to retain a near normal
gait.10 Long-term complications include nonunion and
pseudoarthrosis in up to half of cases. Altered gait with associated
changes in weight distribution lead to premature degenerative changes in
the lumbar spine.10
Various other techniques
treatment options attempted in the setting of periacetabular resection
have included employing megaprostheses, custom-made hemipelvic
prostheses using computed tomography (CT) data, allografts, and
autografts.9,11,12 Although successful results are possible
in this setting, major complications, including significant infection
rates, loosening, dislocation, and fracture, must be taken into
The saddle prosthesis
origination of the saddle prosthesis is credited to Elmar Nieder, a
physician at the EndoKlinic in Hamburg, Germany, in the 1970s. Dr.
Nieder noted that patients with failed hip arthroplasty, including those
with infection or extensive bone loss, had painless articulation
between the neck of the femoral prosthesis and acebatular roof remnants,
despite the loss of the entire acetabular floor. In addition, he
observed that although movement was limited, the configuration was
Making use of these observations, Dr. Nieder devised the first saddle prosthesis.19
It consisted of a novel saddle component mounted directly to a standard
femoral prosthesis component. The term “saddle” referred to the
cranial-most aspect of the prosthesis and how it cradled the residual
ilium along the plane of resection. This implant, the Mark I prosthesis,
was a rigid system with limited intraoperative adjustability (Figure
6). This first iteration was associated with frequent complications,
including fractures, loosening, and cephalad migration.16-18
In the late 1980s, the Mark I was redesigned (Figure 6).19
This new form, known as the Mark II, consists of a saddle, base, and
femoral stem. As with the first iteration, the saddle cups the ilium,
and the femoral stem is implanted in the proximal femoral shaft for
stability. This saddle component has an axis of rotation that rests on a
polyethylene sleeve to maximize range of motion, mainly in hip flexion.
Moreover, the added base component is available in different lengths to
accommodate bone loss and variability in patient anatomy. A set screw
is used to prevent the saddle from dislocating from the base. To date,
this continues to be the most commonly used saddle prosthesis.8,20-23
attempting to restore the iliofemoral weight-bearing axis, this saddle
prosthesis improves stability, minimizes limb-length inequality, and
allows early weight bearing, resulting in improved outcomes. The
addition of swivel points between the prosthesis components allows for
increased functionality. Most importantly, this movement of components
allows rotational motion, decreasing the likelihood of saddle migration
Periacetabular reconstruction prosthesis
most recent variant of the saddle prosthesis is the periacetabular
reconstruction prosthesis (PAR, Stryker Orthopedics, New Jersey) (Figure
7).24 This modified saddle prosthesis utilizes advances in
the design of the total hip replacement. The most significant change in
design fixes the implant to the residual ilium by 3 set screws situated
within the wings of the saddle component. Together, these set screws and
cemented fixation provide approximately 4 times more support than does
the single fixation point of the Mark II prosthesis.24
this iteration of the saddle prosthesis, the ilium is sandwiched
between larger, wider plates. The increased width allows greater
latitude in the level of iliac resection. A captured polyethylene liner
is also placed between the plates and the iliac wing remnant to
compensate for the resectioning of soft tissue and to reduce the chances
of the saddle component dislocating from the iliac wing remnant.
Indications and contraindications of limb-sparing procedures
prostheses are indicated in cases of type II pelvic resection where the
goal is to retain some degree of function. In addition, consideration
is made as to whether the patient’s life expectancy and physical status
justify such a major procedure.8,20-23,25
to saddle-prosthesis placement include recurrence following a previous
limb-sparing procedure, unless the lesion can be entirely resected;
extension through the sacroiliac joint with involvement of sacral nerve
roots and foramina; and extensive soft-tissue infiltration, including
tumor encasement of a neurovascular bundle.8,20-23,25
Osteoporosis is a relative contraindication. The diagnosis of these
contraindications relies upon preoperative imaging studies and specific
patient is placed in the lateral decubitus position and the pelvis is
accessed via the extended iliofemoral approach to allow access to both
the anterior and posterior hip. Mean operative time is 7 hours due to
the complex anatomy as well as efforts to preserve neurovascular and
visceral structures within the operative field. Blood loss can range
from 450 ml to 5000 ml.
The amount of native iliac bone stock for
placement of the prosthesis varies with surgeon preference. The PAR
prosthesis requires more native bone stock than does the traditional
saddle prosthesis, in order to accommodate the larger wings. At our
institution, 2 cm to 4 cm of native bone stock is preferred. In some
instances, the region may need additional bone graft and screw fixation
The margins of the osteotomy depend on the number,
size, and location of the lesions, as well as on the type of neoplastic
disease (Figure 8). In primary neoplastic disease, wide margins can be
curative. The oncologic orthopedic surgeons at our institution use
margins of at least 1 cm of normal tissue, with a range of 1 cm to
5 cm. Margins are less essential in metastatic disease, as the objective
in most of these cases is palliation rather than cure.
the ilium is satisfactorily resected, a notch must be made along the
resection margin to permit fitting of the saddle (Figure 9). Our
surgeons prefer to make this notch along the thickest part of the
remaining ilium. Improper location and preparation can lead to
The Mark II prosthesis articulates with
the ilium without screw fixation. The intent is to allow some movement
at the prosthesis and iliac margin interface for increased
functionality. Some authors claim that this saddle-notch articulation
can cause pressure erosions, superior saddle migration, and premature
failure. Menendez et al claim that their custom saddle prosthesis (PAR)
is less likely to move, as it is fixed with 3 screws. For added
stability, some surgeons place a Gore-Tex artificial ligament (Gore
Medical, Flagstaff, AZ) around the Mark II saddle near the completion of
the procedure. The PAR saddle prosthesis is affixed to the ilium with 3
screws and is believed to be less likely to migrate.
tension consists of the tendinous attachments of the psoas and the
abductors, and it has been found to be critical for postoperative
stability. To help preserve the native mechanical forces in the hip,
involved muscles are sutured directly to the distal residual bone.
Intraoperative evaluation consisting of range of motion, saddle
stability, and ease of dislocation is then performed. A modular
intermediate component allows for patient variability to maintain the
soft-tissue tension (Figure 10).
At our institution, additional
measures are taken in placing the PAR saddle prosthesis. The space
posterior to the prosthesis is packed with a sterile, absorbable gelatin
sponge (Gelfoam, Pfizer, New York, NY). The space between the
prosthesis and the ilium is packed with polymethylmethacrylate (PMMA).
The femoral component is implanted, as in a total hip replacement.
Modular components are placed as needed to maintain anatomic length and
promote mechanical stability. These measures have significantly reduced
postoperative loosening and migration compared to other prostheses and
Postoperative imaging technique
and postoperative conventional anteroposterior (AP) and lateral
radiographs are used to demonstrate prosthesis placement. In some cases,
additional obturator, oblique (Judet), and alar views help to visualize
saddle placement and the interfaces between the prosthesis components
and the bone (Figure 11).
Our suggested conventional radiographic
technique for evaluating saddle positioning consists of AP standing
pelvis, as well as AP and frog-leg lateral views of the femur. Oblique
(Judet) views of the pelvis are optional. A fluoroscopy-guided oblique
film of the pelvis helps to profile the interface of saddle and bone. We
have termed this an “interface view.” Under fluoroscopic guidance, the
patient is obliqued to a position optimized to demonstrate the
prosthesis-ilium interface. The patient is secured in this position with
pillows, and an overhead radiograph is then obtained. This provides
more detail than a fluoroscopic spot image and also avoids streak
artifacts associated with CT.
In some cases, CT imaging may be
needed to assess saddle apposition to the ilium and its relation to
intrapelvic structures. Streak artifact may make interpretation
difficult, thus positioning and technique are essential. Magnetic
resonance imaging (MRI) may be difficult to interpret due to signal loss
from artifact. Indications for MRI include evaluating recurrence of
tumor or complications, such as infection. Our preferred protocol
consists of axial and coronal T1-weighted and short tau inversion
recovery (STIR) sequences. Chemical shift fat-saturation techniques are
generally avoided, as they are more vulnerable to field inhomogeneities
related to the metallic hardware. Postcontrast T1-weighted sequences
following the intravenous administration of a gadolinium-based agent can
be helpful to demonstrate soft-tissue enhancement, which may indicate
recurrent or residual tumor in areas not obscured by artifact.
evaluation of a saddle prosthesis is much like that of any implanted
hardware. The alignment, position, and apposition of the prosthesis to
adjacent cement and bone are important to observe and report. Close
apposition of the saddle and adjacent iliac resection margin, as well as
the femoral stem and adjacent residual femoral cortex, are essential to
identify. Component alignment with respect to bone, as well as to each
other, is also important to evaluate (Figures 11 and 12).
Imaging evidence of complication
potential complications may be evident on imaging. As with any
orthopedic hardware, perihardware lucency may indicate loosening or
infection. Subtle loosening can be better assessed by comparing images
to prior radiographs. Signs of infection are modality dependent and can
include soft-tissue or perihardware gas, fluid collection with or
without enhancing margin in the setting of contrast, and increased
neocapsular fluid volume.26 An ultrasound or arthrogram may
be performed to evaluate for evidence of intra-articular infection, such
as a distended joint space. Infection may also present as cephalad
migration and dislocation or rapid loosening. Rates of infection with
the earlier saddle prostheses reached as high as 20%.19 In
cases where CT or MRI fail to clarify a possible infection, a nuclear
medicine study, such as a “triple tracer” scan (three-phase
technetium-99m-labeled bone scan, indium-111-labeled white blood cell
scan, and technetium-99m-labeled sulfoid colloid scan) may be of value.27
Cephalad migration, also referred to as “bedding in,” is quite common with the Mark I and Mark II prostheses (Figure 13).19
Upward migration is best evaluated on successive films by measuring the
distance between the base of the intermediate component and a
straight-line tangential to the inferior border of the sacroiliac
joints.19 Sclerosis at the saddle-iliac interface is common.
If no sclerosis is noted, increased cranial migration can occur.
Significant or sudden changes in the rate of migration may indicate
infection (Figures 14 and 15).
Dislocation can occur between the
ilium and the saddle component, as well as between the components
themselves (Figures16 and 17).20 Subtle changes in angulation
may signal impending dislocation. Cephalad migration or dislocation may
call for MRI to evaluate for possible nerve injury, especially to the
femoral and sciatic nerves.
Focal linear lucency within the
hardware may indicate hardware fracture, especially after a fall or
other similar post-traumatic presentation. In addition, potential
perihardware fracture should also be assessed. Regions particularly at
risk include the femoral stem tip as well as the iliac wing margin
(Figure 18). Signs of accelerated prosthesis wear, such as focal surface
erosion or the presence of fine metal particles, should also be
identified and reported, as this may lead to complete hardware failure.28
bone formation can be seen around the saddle prosthesis (Figure 19).
Some orthopedists regard this as a desired outcome, believing that it
provides a better foundation for prosthesis seating. Others regard it as
evidence of malposition and abnormal distribution of mechanical forces.
Heterotopic ossification resulting in bridging across a joint or
limiting motion is considered a complication.29, 30
of recurrent primary or metastatic osseous disease should also be
evaluated. Interval occurrence or progression of abnormal mineralization
may indicate recurrence or residual disease. Prosthesis displacement
without identifiable cause on conventional radiography may also indicate
recurrence. Cross-sectional imaging, such as CT or MRI, may better
indicate recurrence and may also demonstrate a soft-tissue component
(Figures 20 and 21).
Patient recovery and function
previously noted, periacetabular resection and saddle prosthesis
placement is a complex procedure that can involve a large anatomic
region. Accordingly, recovery time varies and hospitalization can be
quite long.31 One group reported a median postoperative hospital course of 6 weeks,8
while at our institution the median postoperative course is a bit
shorter at 3 to 4 weeks. The patient can be immobilized in traction or
abduction from 5 days to 2 weeks. Average time to ambulation varies from
to 9 months with extensive physical
Our oncologic orthopedists have observed an average time
to ambulation of 3 weeks when the periacetabular reconstruction
prosthesis is employed. Initially, most patients require a cane or
walker; they ambulate with the Trendelenburg gait, in which the pelvis
sags opposite the affected side during weight bearing on the affected
side. Compensation is achieved by leaning the torso toward the affected
In general, saddle prosthesis placement improves analgesia
requirements, patient independence, and ambulation. Most patients are
able to ambulate with a cane and achieve an improved quality of life.
neoplastic disease, both primary and metastatic, can involve the
pelvis. In cases for which surgical resection is elected and the
periacetabular region is involved, a saddle prosthesis may allow
limb-sparing surgery and preserve reasonable postoperative
functionality, including improved postoperative functional status,
decreased limb length discrepancy, and reduced analgesia requirements.
Understanding the radiographic appearance of a normal prosthesis and
recognizing early imaging evidence of potential postoperative
complications are important to reduce or prevent patient morbidity.
Normal postoperative imaging should demonstrate close apposition of the
saddle component to the segment of residual ilium. Postoperative
complications, including saddle cephalad migration, saddle dislocation,
fracture, and tumor recurrence, can occur. Continued refinement of
prosthesis design and surgical technique will hopefully improve success
rates and allow this technique to be applied to more patients with
- Frassica FJ, Sim, FH. Pathogenesis and prognosis. In: Sim FH, editor. Diagnosis and management of metastatic bone disease: A multidisciplinary approach. New York, NY: Raven Press; 1988;1-6.
- Yasko AW, Chow, W. Bone sarcomas. In: Padzur R, Coia LR, Hoskins WJ, Wagman LD, editor. Cancer management: A multidisciplinary approach. 10th ed. Darien, CT: CMPMedica; 2007.
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- Fuchs B, O’Connor MI, Kaufman KR, et al. Iliofemoral arthrodesis and pseudarthrosis: A long-term functional outcome evaluation. Clin Orthop Related Res. 2002;29-35. PubMed PMID: 11953592.
- Windhager R, Karner J, Kutschera HP, et al. Limb salvage in periacetabular sarcomas: Review of 21 consecutive cases. Clin Orthop Related Res. 1996:265-276. PubMed PMID: 8895649.
- Zehr RJ, Enneking WF, Scarborough MT. Allograft-prosthesis composite versus megaprosthesis in proximal femoral reconstruction. Clin Orthop Related Res. 1996;322:207-223. PubMed PMID: 8542698.
- Donati D, Zavatta M, Gozzi E, et al. Modular prosthetic replacement
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