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Rapid Prototyping for Periacetabular Osteotomy in the Teenager and Young Adult
Richard M. Schwend, Carlos Rio, Brent Milner, George Brown
Introduction
The Bernese peri-acetabular osteotomy, also known as the Ganz peri-acetabular osteotomy is a
well described joint-preserving reconstructive procedure utilized after closure of the tri-radiate
cartilage (Ganz et al 1988). The surgical goal is to improve the acetabular position and coverage
of the femoral head in cases of residual or late diagnosed developmental hip dysplasia in
adolescents and young adults (Siebenrock et al 1999, 2001, Leunig et al 2001, MacDonald et al
1999, Murphy et al 1999, Leunig et al 1998, Hipp et al 1999). The functional goal is to relieve
pain and instability and to preserve the life of the hip joint (Siebenrock 1999).
One of the more exacting aspects of the operation is the making of the pelvic bony cuts using
angled chisels. Current technique favors an abductor sparing approach, utilizing intra- pelvic
ostotomies (Murphy, Millis 1999). Five separate bony cuts are made around the acetabulum: 1.
anterior ischium, 2. pubis, 3. innominate, 4. along posterior column, 5. quadrilateral surface (Ganz
et al 1988). These cuts leave the posterior column intact, are close enough to the acetabulum to
allow ease of rotation and translation, but leave enough of the soft tissues attached to preserve the
blood supply from the superior and inferior gluteal vessels and the acetabular branch from the
obturator artery (Beck 2003, Hurson 2004).
Another aspect of the operation is fragment positioning. In a series looking at the first 75 Bernese
periacetabular osteotomies performed by Ganz, 73% of the hip joints were preserved with good or
excellent results (Siebenrock 1999). Their results increased to 88% good and excellent if only
Tonnis grade 0-1 hips without known labral lesions were studied (Siebenrock 1999). Their series
showed the need for very accurate positioning of the fragment. Under correction leads to
inadequate coverage and perstent dysplasia; overcorrection results in impingement syndrome (need
reference to impingement syndrome).
This is a technically challenging procedure with a prolonged learning curve (Hussel et al 1999). In
the Ganz series, most of the major problems occurred in the first 18 cases (Siebenrock et al 2001).
These included intra-articular osteotomy, acetabular osteonecrosis, a need for repeat osteotomy,
femoral nerve palsy, extensive lateralization of the fragment and subsequent femoral head
subluxation. Matta (1999) felt that the very steep initial learning curve could cause poor fragment
positioning such as excessive lateralization, retroversion, too medial placement and ante-version.
Davey et al (1999) found less complications with increasing surgeon experience. They noted a
complication rate of 17% in the first 35 cases, which decreased to 2.9% in the second 35 cases.
Besides the shape and direction of the acetabulum, the entire innominate bone is misshapen,
probably as a result of growth disturbance of the tri-radiate cartilage (Albinana et al 1995). The
technical challenges of blind cuts, steep learning curve, unique and variable pelvic anatomy
suggest a need for technological aids to make the surgery more exact and safe.
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Computer assisted surgery (CAS) has been shown to be a practical method of surgical planning
and navigation in orthopedic surgery (Brown et al 1999, 2000, 2001, 2003, Glossop 1998, Kahler
1997, 1998, Richter et al 2000, Tessman et al 1996). CAS technology has also been applied to the
Bernese peri-acetabular osteotomy (Langlotz et al 1997, Langlotz et al 1998, Dutiot et al 1999).
Langlotz et al (1997) used intra-operative computer assistance in twelve peri-acetabular
osteotomies. They noted increases in blood loss and surgery time but increased accuracy and
safety, with decreased intra-operative radiographs. The evolution of CT imaging and intraoperative CAS leads one to the conclusion that advanced imaging, computer modeling, and intraoperative assistance can lead to increased accuracy. However CAS has the drawbacks of expense
and the potential for increased operative time and bleeding.
Surgical applications using evolving the technology of sterolithography (rapid prototyping) have
been explored in specialty areas such as maxillo-facial surgery (Kelly 1998, Keman et al 2000) and
orthopedic applications such as treatment of calcaneal and acetabular fractures (Migaud et al 1997,
Thomas et al 2000, Kaci et al 1997, Brown 2002). Radermacher et al (1998) discussed image
based individual templates for various surgeries, including peri-acetabular redirectional triple
osteotomy. They believed that this technology could be simple, economical, allow precise
preoperative planning, avoid continual fluoroscopic monitoring, involve less intra-operative
radiation, while greater accuracy and safety achieved. Brown et al (2003) demonstrated the
evolving role for rapid prototyping in trauma surgery.
In a cadaveric study of ten hip undergoing periacetabular osteotomy we introduced rapid
prototyping technology from CT data to the Bernese peri-acetabular (Schwend et al 2002). The
protocol used 3-D CT reconstructions of pelvic anatomy, preoperative surgical planning of periacetabular bony cuts and fabrication of individualized intra-operative templates for accurate
placement of these cuts with minimal dissection. Accurate placement of the pelvic cuts was
obtained in each of the ten specimens with no use of intra-operative radiation. There was no
violation of the hip joint or posterior column. The cuts allowed for adequate rotation and
medialization of the free fragment. Computer assisted surgical planning with computer generated
templating appeared to be an accurate method to guide placement of the bony cuts with minimal
intra-operative imaging.
The purpose of this current clinical study was to demonstrate some of the benefits of rapid
prototype modeling and templating for assistance in performing the Bernese periacetabular
ostetomy. Anticipated advantages may include: ability to show the patient the planned procedure
for better informed consent, a better appreciation of the underlying pathology, improved surgical
planning, quicker learning curve for the surgeon, less intra-operative radiation, more accurate bone
cuts, and reliable acetabular segment positioning.
Methods
Preoperative Imaging and Model Construction. Seven patients with functionally significant and
painful acetabular dysplasia received standing anterior-posterior, Von Rosen, and standing false
profile plain roentgenograms of the pelvis. Lateral and anterior center edge angle, angle of the
weight bearing surface of the acetabulum, and the distance of the medial aspect of the femoral
head from the vertical line from the inner aspect of the pelvis were measured. Thin cut (3mm)
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spiral CT scan of the supine pelvis were obtained using a Picker CT scanner. 3-D images of the
pelvis were created using the Picker 2000 workstation. The images were transferred via Exabyte
820 tape drive to a personal computer. The 3-D reconstructions were loaded into the Materialise
Interactive Medical Imaging Control System (MIMICS 6.3) software package (Materialise USA,
Ann Arbor, MI), which then interfaced with a computer assisted design (CAD) workstation. Rapid
stereolithographic of pelvic anatomy was performed using a Thermojet 3-D Printer (3-D Systems,
Valencia, CA) to convert the CT images into a life-size plaster model. The acetabular fragment
orientation was noted on all models. The acetabular volume was measured on specimen models by
filling the acetabulum with water. The pelvic model, cutting template, proposed osteotomies and
anticipated fragment position were shown to the patient during the pre-operative consultation.
Risks and benefits of surgical correction were explained and informed consent for the procedure
was obtained.
Preoperative Planning Appropriate bony cuts for the Bernese peri-acetabular osteotomy were
outlined on the pelvic model. A custom molded implant grade methylmethacrylate template was
created for the hemi-pelvis (figure 1) to guide the intra-operative placement of the angled chisels
used during the operation. The desired bone cuts were planned to assure avoidance of intraarticular osteotomy and to provide adequate rotation and fixation of the free acetabular segment.
The planned osteotomy cuts are made on the pelvic model and the free acetabular fragment is
placed in the desired position based on the anticipated amount of correction needed. Surgical goal
was to correct the sourcil to a horizontal position (Hussell 1999) and the lateral center edge angle
to 30 degrees. An AP radiograph of the plaster model, before and after fragment positioning can
be obtained to assure adequate positioning of the fragment and that the sourcil is horizontal
(Figure 2).
Surgical Procedure. The surgery is performed through a Smith Peterson anterior abductor sparing
approach with the supine patient under general and epidural anesthesia. The first osteotomy cut is
made using fluoroscopic guidance, while the second through fifth cuts can be made with the
template placed on the inner aspect of the pelvis for guidance. Once the acetabular fragment is
free, the acetabular fragment is positioned in the same degree of correction that was determined on
the pre-operative pelvic model. Once accurately positioned, the acetabulum is fixed to the pelvis
with screw implants and radiographs obtained. The capsule was routinely opened to investigate
the labrum, and to check for full range of motion with no rim impingement. Partial weight bearing
crutch assistance was used for six weeks, protected full weight bearing for six more weeks, then
full weight bearing walking. Sports activities were allowed when strength has returned, typically
at six months.
Results
Seven patients, age range 12.5 -30.5 years, with high grade acetabular dysplasia were evaluated
with rapid prototyping. Five patients had DDH and 2 had acetabular dysplasia secondary to
Charcot Marie Tooth disease (Table 1). Blood loss ranged from 100-1200 cc with and mean of
700 cc. Follow-up ranged from 22-59 months with an average of 45 months. There were no intraarticular osteotomies, severe blood loss or infection. Patient number four with Charcot Marie
Tooth disease (CMT) had a transient femoral nerve palsy that resolved and patient number five,
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also with CMT, had a transient sciatic nerve palsy that also resolved within several months. There
were no permanent complications. Two patients have received post operative modeling that shows
the fragment position to match the planned preoperative position (Figure 3).
Radiographic results (Table). All patients resolved their radiographic dysplasia. The lateral center
edge angle improved to a normal range without excessive correction. There was no statistical
difference between the corrected lateral center edge angle (mean 28.1 deg, range 25-30 deg) and
the surgical goal of 30 deg. The femoral head medialized on the post operative radiograph by a
mean of 8 mm. The weight bearing surface angle improved from 27 to 14.5 degrees. All patients
postoperative had a normal Shenton line. There were no non-unions, radiographic evidence of
osteoarthritis, or implant complications.
Discussion
There are numerous technical challenges and potencial hazards involved in performing the
Bernese periacebular osteotomy. These include: avoiding intraarticular ostetomy, acetabular
osteonecrosis, under correction and overcorrection, avoiding excessive blood loss, nerve injury and
non-union. There is a definite learning curve involved, estimated to require approximally 8-35
cases (Siebenrock 1999, Davey 1999) that must be navigated early in one’s series. This is
accomplished by spending time visiting with an experienced surgeon, cadaver laboratory
experience performing the procedure with dissection of the result and time examining human or
plastic pelvices. The purpose of our study was to develop a physical aid to assist the surgeon to
visualize the pathology and to diminish some of the technical problems seen early in ones’s
learning curve.
Less than optimal postoperative acetabular index and less anterior coverage correction are some of
the factors leading to an unfavorable outcome (Siebenrock et al 1999). They believed that the
correct intra-operative positioning of the reoriented acetabular fragment to be the most difficult
part of the operation. They further stressed that there should be no intra-operative guesswork.
Intra-operative radiographs can be misleading and a number of parameters need to be evaluated
rather than an isolated radiographically measured angle. When measuring the CE angle, it is
important to use the lateral aspect of the bony acetabulum rather than the lateral aspect of the
sourcil (Kim et al 2000). The correction must be precise, the extent of the direction individualized
to the patient in order to optimize stability and kinetics. Deficiencies in coverage must be
corrected without creating reciprocal pathologic deficiency (Hussell 1999). To avoid
overcorrection, an intra-operative, neutral position of the sourcil over the femoral head is preferred
over a normal center edge angle (Hussell 1999). The femoral head should be positioned about five
mm lateral to the ilioischial line. Flouroscopy has been found to not be adequate to properly
position the fragment. According to Hussell et al “There really exists, for each individual, only
one optimal correction” (1999). Under-corrections occur despite the radiograph giving a (false)
impression of adequate correction. In our study all patients achieved the surgical goal of a
horizontal sourcil and a lateral center edge angle that was statistically within the surgical goal of
no more than 30 degrees (mean 28, range 25-30 degrees).
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To ensure proper acetabular positioning Ganz emphasizes obtaining an intra-operative pelvic
radiograph to place the fragment in the best possible position (Leunig et al 2001). Despite a
normal appearing intra-operative radiograph, Ganz reported that 29% of patients had a postoperative impingement test, suggesting that intra-operative overcorrection of the acetabulum
(Myers et al 1999). It appears that the range between insufficient, normal and over-coverage is
rather small and is influenced by a multitude of factors (Siebenrock et al 1999). Cockarell et al
(1999) avoided over correction by opening the joint to look for clinical impingement with hip
flexion, ensuring 100 degrees post-operative hip flexion, and that no more than 33% of the femoral
head surface on an AP film should be covered by the anterior acetabulum. Problems with undercorrection and over-correction of the acetabulum suggest a need for advanced imaging and
modeling of the correction. Haddad et al (2000) used 3D CT for measuring the coverage of the
femoral head in acetabular dysplasia in adults. Ten patients with symptomatic dysplasia had CT to
assess the degree and direction of dysplasia and to measure the coverage obtained at acetabular
osteotomy. CT was determined to be more clear than plain radiographs. Thus use of CT imaging
can increase imaging accuracy.
We noted in all of the models a consistent egg shape appearance of the acetabulum with anterior
lateral orientation. Evaluating all the pelvic models at one time demonstrated a remarkable
similarity in shape and orientation of all deficient hips. Volume measurements showed reduced
acetabulum volume in the patients with unilateral acetabular dysplasia (average of 15.5 cc for the
four patients with DDH and acetabular dysplasia compared to 25.5 cc for the two patients with
Charcot Marie Tooth disease. We postulate that patients with Charcot Marie Tooth disease had
relatively normal acetabular development earlier in life and had slow subluxation with acetabular
deformity, leading to a relatively larger acetabular volume. These relationships became very
obvious with the actual model in hand. After the bony cuts were placed on the pelvic model, it
was very straightforward to position the fragment into the position that best matched the other hip
if it was normal, a radiograph of the model could be obtained with the fragment in the corrected
position, and the fragment position then stabilized. During the actual surgical procedure it was
routinely possible to accurately match the acetabular position with that determined on the model.
We used the cutting templates as a guide to the osteotomies and avoided intraoperative imaging for
the fourth cut along the posterior column. Intra-operative fluoroscopy was still used to make the
first blind ischial cut.
In conclusion preoperative planning with rapid prototype pelvic models has several advantages. It
allows the surgeon to appreciate the size, shape and orientation of the dysplastic acetabululum.
Cutting templates can be molded if desired to assist in creating the intraoperative osteotomies. The
patient and family can be shown the pathology and the anticipated surgery for more thorough
informed consent. Assessment of the acetabular position can be made on the preoperative model
and matched very accurately during the surgical procedure, avoiding many of the risks of under
and over-correction. Although not proven in this study, use of the cutting template and advanced
knowledge of the expected position of the acetabulum should lead to less intra-operative imaging
and to less operative time and bleeding. We believe that these benefits of rapid prototype
modeling should assist the surgeon in overcoming some of the technical aspects of the initial
learning curve for this complex surgical procedure.
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_______________________________________________________________________________
Table 1 Data on the Patients
Case Age
Sex
(years)
Hip
Diagnosis
Lat CE
Angle
(deg)
Ant CE
Angle
(deg)
Distance
(mm)
WB
angle
(deg)
Acetabular
Volume
(mm3)
(Post op values are in parenthesis)
______________________________________________________________________________
1
14-7
F
L
DDH
2 (25)
10
18(16)
27( 20)
15.4
2
12-6
F
L
DDH
-6 (29)
0 (24)
12(-3)
24(13)
13.1
3
30-6
F
R
DDH
8 (26.5)
0 (30)
11( 7)
28(16)
17
4
14-10
M
R
CMT
7 (28)
-7 (29)
16 (3)
20(12)
29.6
5
16-3
F
R
CMT
0 (30)
0
21 (0)
21(11)
21.4
6
13-0
F
L
DDH
-11(30)
-5
17(12.5) 32(15)
16.8
7
16-9
F
L
DD
0 (31)
0 (27)
15 (5)
43 (9)
Mean
0 (28.1) -0.3
13.9(5.9) 27(14.5)
SD
7.4(2)
5.3 (7.3) 4(3.3)
5.9
10
14
______________________________________________________________________________
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Figure Legends
Figure 1. Template for guiding the osteotomy cuts. Template is used to guide cuts on the model
as well as being used an intraoperative guide during the surgical procedure. Note positioning of
osteotome along posterior column cut.
Figure 2a. Patient 2. Preoperative plain standing roentgenogram. Lateral CE angle is -6 deg,
distance of medial aspect of the femoral head is 12 mm, angle of weight bearing surface of
acetabulum 24 degrees.
Figure 2b. Preoperative plaster model before correction.
Figure 2c. Preoperative plaster model before correction. Close up of egg shaped acetabulum.
Figure 2d. Preoperative radiograph of model before correction
Figure 2d. Plaster model after correction and placed in desired position
Figure 2e. Radiograph of model after position.
Figure 3a. Patient 2. Postoperative plaster model.
Figure 3b. Postoperative radiograph of model.
Figure 3c. Postoperative standing AP pelvis. Lateral CE angle is 29 deg, distance of medial aspect
of the femoral head is -3 mm, angle of weight bearing surface of acetabulum 13 degrees.
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Schwend Revised July 2007
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