Understanding Complex Patho-Morphology - University of Utah ...

Hip Preserving Surgery:
Understanding Complex
Patho-Morphology
*Christopher L. Peters MD
Jill Erickson PA-C
Lucas Anderson PA-C
Andrew Anderson PhD
Jeff Weiss PhD
University of Utah Orthopaedic Center,
Salt Lake City, Utah
Disclosures are listed in the AAOS
program and website.
No conflicts with this subject.

Introduction:
Recent evidence suggests that abnormal hip morphology may be the
primary etiology of hip osteoarthritis in young adults. Hip pathomorphology
is manifested as acetabular deficiency or malorientation, or as
femoral deformity and malorientation, or, most commonly, combinations
of the two. Contemporary hip preservation surgical intervention has been
directed at correction of these malformations and associated chondrolabral
injuries and has shown promise in alleviation of hip pain and possibly
retardation of osteoarthritis progression.
With the increasing number of available of surgical methodologies
directed at hip preservation such as surgical dislocation, osteochondroplasty,
hip arthroscopy, and redirectional acetabular osteotomy, the
importance of understanding the pathological process driving the painful
hip has become paramount. In an effort to augment the basic information
obtained from clinical examination, two-dimensional plain radiography,
and magnetic resonance arthrography, we have utilized a validated 3D
modeling protocol to serve as a diagnostic and surgical planning tool for
hip preservation surgery. Three-dimensional modeling has helped
emphasize the complex pathomorphology evident in many patients with
hip dysplasia and femoroacetabular impingement and may have a future
role in classification and treatment of hip maladies in young adults.
Objectives:
1. Describe the typical presentation of the young adult with a painful hip
and corresponding case examples which illustrate common morphologic
abnormalities of the femur and acetabulum.
2. Using case examples, illustrate the value of a comprehensive imaging
protocol to facilitate diagnosis and treatment of patients with FAI and
dysplasia.
3. Present our experience with 3D model development as a function of, or
subset of work focused on the biomechanics of the dysplastic hip.
4. Present the experience with subject-specific models including the
exhibition of combined acetabula and femoral deformities.
5. Outline future work including development of a 3D classification
system.

Femoral Deformities
20 year old male, college football player, with acetabular retroversion, positive posterior wall sign and an Os acetabuli presented with
symptomatic FAI. Treatment consisted of Surgical Dislocation, rim debridement, Os excison, labral refixation and femoral head-neck
osteochondroplasty.
23 year old male with history of prior femoral neck fracture treated with ORIF with resultant femoral retroversion deformity and FAI. Treatment
consisted of surgical dislocation, delaminated hyaline cartilage resection, rim debridement and labral refixation.
14 year old female with history of Perthes disease and FAI. Treatment consisted of surgical dislocation with relative femoral neck lengthening.
Acetabular cartilage was well maintained (OB1) but the labrum was detached at 12:30. Acetabulum received no treatment, femoral head-neck
offset improved with osteochondroplasty.
Acetabular Deformities
27 year old male presented with FAI and restricted ROM of the right hip. Radiographs revealed acetabular overcoverage and retroversion from
calcified labrum. Treatment consisted of surgical dislocation, resection of calcified labrum from 8 to 4 o’clock.
27 year old surfer who sustained a “hip injury” diving, presented with symptomatic FAI with acetabular retroversion, positive posterior wall sign,
and an os acetabuli. CT scan and 3D model confirmed deficient posterior wall. Treatment consisted of periacetabular osteotomy with femoral
head-neck osteochondroplasty, removal of os and labral repair.
43 year old female presented with restricted ROM and “a labral tear”. Radiographs revealed coxa profunda. MRA suggested labral tear.
Treatment consisted of surgical dislocation and rim resection with labral refixation from 3-9 o’clock.
30 year old female presented with symptomatic classic acetabular dysplasia worse on the left. Treatment consisted of left periacetabular osteotomy.
A year later she developed worsening symptoms on the right, and we performed a right PAO with femoral head-neck osteochondroplasty.
She is pain-free at 4 and 5 year follow-up.

Contemporary Preoperative Planning:
18 year old male previously treated for FAI of the left hip with surgical dislocation and femoral offset improvement.
The right hip exhibited acetabular retroversion with a positive posterior wall sign and visible ischial
spine. 3D modeling confirmed significant acetabular posterior wall deficiency and intact hyaline cartilage.
Treatment consisted of anteversion producing PAO and femoral head-neck offset improvement. Hip pain and
function is improved at 4 years follow-up. Full video of the progression and surgeries can be seen below.
18 year old male with FAI and history of Perthes disease. MRA showed evidence of loose osteochondral
bodies within the joint. CT arthrography and 3D modeling confirmed typical Perthes femoral deformity
and osteochondral head fragmentation. Treatment consisted of relative femoral neck lengthening and
grafting of the femoral head defect with osteochondral tissue resected from the head-neck junction.
Acetabular hyaline cartilage delamination (OB4) was treated with resection and labral refixation. Hip
pain and function is improved at 3 year follow-up.

Three-Dimensional Model Development and Validation:
Overview : Preoperative surgical planning protocol (flowchart) developed in parallel with
ongoing NIH grant (#RO1AR053344). Grant objective is to compare cartilage mechanics
between normal and dysplastic hips using patient-specific finite element analysis (FEA)
Clinic Visit,
Preliminary
Diagnosis
1.Assess Exclusion Criteria
2.Obtain Informed Consent
3.Schedule CTA
Patient Recruitment
CTA
• FAI (cam, pincer), classic dysplasia, 18-40 years old
• Informed consent obtained (IRB#10983)
• Excluded : Tonnis >1, pregnant women, THA candidates
CT Arthrography
• 15-30 ml contrast injected into capsule under fluoroscopic
control (2:1 lidocaine 1% to Omnipaque 300)
• With needle removed, traction applied to ensure contrast
covered proximal femur essential (Figure 1)
• Multi-Detector CT scan covered proximal iliac crest to trochanter
and distal condyles (Figure 2)
• Femur hare traction splint affixed to affected limb, ~50-80 lbs.
force delivered to increase joint space; traction was a
necessary precursor to obtain quality images (Figure 3)
• Settings (Figure 2)
– Pelvis 120 kVP, CareDose [200-400 mAs], 1 mm slice thick. w/o
overlap, FOV = 250-400 mm, resolution ~0.75 x 0.75 x 1.0 mm
– Femoral Condyles 80 kVP, 100 mAs, 3 mm slice thick. w/o overlap,
FOV = pelvic scan
Patient-
Specific
3D
FEA
Reconstruction Analyze 3D
Morphology
Figure 1: Fluoroscope images of patient
without (left) and with (right) manual traction
Contrast
Traction
Predict Long-Term Outcome
1. FE models yield contact area,
peak pressure, average pressures,
shear stresses
2. Pre/PostOp FEA
3. Develop improved classification
system (FAI, Retro, Dysplasia)
All Inclusive PreOp Report
1. Radiological, MRA/CTA findings
2. Anatomical views of 3D model
3. Cartilage thickness & asphericity
of femur reported as color 3D plot
Figure 2: CT Scout of Study Subject
Pelvic
Cartilage
Traction
Contrast
Femur
Cartilage
Figure 3: Coronal CTA images of patient without (left) and
with (right) traction applied. Contrast clearly covers the entire
proximal femur in the hip under traction, improving image
quality and ability to delineate pelvic from femoral cartilage.

3D Reconstruction & Validation
• Separate surfaces for cortical bone and cartilage semi-automatically reconstructed using commercial software (Amira 5.2, Visage Imaging
Inc.) (Figure 4)
• Requires approximately 8-12 hours of segmentation time for each model (aided by undergraduate research assistants)
• Surfaces smoothed and decimated to remove staircase artifact (Figure 4)
• Surfaces used for preoperative surgical planning (see below)
• Validation
– Error between true anatomy and 3D reconstruction shown to be < 3% (1)
– Cartilage >1.0 mm thick can be accurately (i.e. < 10% error) reconstructed from CTA images (2)
– 2:1 dilution factor for contrast agent allows for accurate segmentation (2); not too bright / dull
Amira 5.2 Console
Figure 4: 3D models are generated from CT arthrography image data on a patient-specific basis using Amira 5.2. Example of process for patient with classic dysplasia.
Left) Screen shot of program showing the outline for the pelvic cortex (coronal plane). Segmented splines are triangulated into a surface (middle) which is smoothed
and decimated to remove digitization and CT stair-case artifacts (right).
Data Analysis & Comprehensive Preoperative Surgical Report
• Validated thickness algorithm (1) estimates cartilage thickness on a patient-specific basis (Figure 5)
• Femur surface defining interface between bone/cartilage fit to perfect sphere using validated protocol (3) (Figure 6)
• In special cases, life sized prototype made in-house using 3D printer (~$150 for femur + hemi-pelvis)
• Preoperative report includes
– Plain films and measurements (CEA, wall sign, cross-over sign, etc.)
– CTA (degree of retroversion, lesions, labral tears, angles, etc.)
– Screenshots of 3D model (full appreciation of anterior/posterior over/under-coverage)
– Plot of cartilage thickness (Figure 5)
– Localized asphericity of femoral head (Figure 6)
Figure 5: Plot of cartilage thickness in a normal human femur (left) and
acetabulum (right). The reconstruction error for cartilage has been
shown to be < 10% if cartilage is at least 1.0 mm thick (2).
Surface Triangulation
Patient-Specific Finite Element Analysis (ongoing work)
• Validated finite element analysis (FEA) protocol applied to generate patient-specific FE meshes (4)
– Varying cartilage and cortical bone thickness (Figure 7)
• Patient-specific kinematics acquired from motion analysis lab and applied to FE models
• FE analysis yields patient-specific cartilage contact pressures, contact area, shear stresses, etc. (Figures 7, 8)
– Compare normal vs. dysplastic hip joints (Figure 8) to develop a quantitative understanding of how geometrical deformities predispose
the hip to early onset OA
Femur
Cortex Femur
Cartilage
2.0 mm
0.8 mm
7 mm
0.2 mm
Figure 7: Patient-specific FE mesh of a normal human hip. Left) detail of femur FE mesh showing cortical bone and cartilage.
Cartilage has been modeled with varying thickness as segmented from the CT images. Middle) cortical bone is modeled using
shell elements with position dependent thickness, which is also based on the CT image data. Right) magnitude and
distribution of cartilage contact pressures in a normal acetabulum under simulated descending stairs.
4 mm
0.2 mm
Smoothing,
Decimati0on
Rough Triangulated Surface Smoothed and Decimated Surface
Cam Patient
Deviation from Sphere
Figure 6: Plot of asphericity of femoral head. Left) cam FAI patient. Middle) normal joint. Right) when
re-scaled, normal joint continues to show very little deviation from perfect sphere. Note- positive
deviations = “bone to be removed” whereas negative deviations = bone indentations. Please see
ORS2009-3159 for more details.
7 MPa
0 MPa
Normal
+1.0 mm
-1.0 mm
Normal
Dysplastic
Deviation from Sphere (updated range)
+0.05 mm
Normal
-0.05 mm
Walking Stair-Climbing Descending Stairs
10 MPa
0 MPa
Figure 8: Comparison of cartilage contact pressures between a
normal (top) and dysplastic (bottom) hip joint. Pressures remained
elevated in the dysplastic joint during simulated walking, stairclimbing
and descending stairs (columns).

Three-dimensional Imaging and Complex Pathomorphology:
17 year old male with profound acetabular retroversion presented with symptomatic FAI. 3D modeling
clearly demonstrates posterior acetabular wall deficiency and relative lack of head-neck offset. Treatment
consisted of anteversion producing PAO with femoral head-neck offset.
24 year old male with severe bilateral classic acetabular dysplasia. 3D modeling clearly demonstrates
anterior and lateral socket deficiency. Treatment consisted of PAO and femoral osteochondroplasty.
35 year old male with combined cam and pincer FAI. 3D modeling demonstrates acetabular retroversion
and poor femoral head-neck offset.Treatment consisted of surgical dislocation, excision of delaminated
articular cartilage from 1-3 o’clock, labral refixation and femoral offset improvement.
23 year old male with combined cam and pincer FAI. 3D modeling demonstrates acetabular retroversion with
prominent Os acetabuli and posterior deficiency. Treatment consisted of periacetabular osteotomy, Os removal,
labral repair and femoral head-neck osteochondroplasty as seen in the fluoroscopy pictures below.

Conclusions:
1. Abnormal hip morphology may be the dominant cause of hip
pain and the subsequent development of osteoarthritis of the hip
in young adults.
2. Hip pathomorphology may be manifested by femoral or acetabular
deformity or malorientation. Most patients have combinations
of the two.
3. Surgical intervention should be focused on correction of the
underlying pathomorphology as well as the resultant chondrolabral
tissue damage.
4. 3D computational modeling of the hip can facilitate proper identification
of the major pathomorphologic anatomy.
5. 3D computational modeling may facilitate improved understanding
of FAI and dysplasia of the hip via new classification systems
and verification of treatment methodologies.
Acknowledgement: NIH Grant #R01AR053344
References:
1. Anderson, A.E., Peters, C.L., Weiss, J.A. et al. Journal of Biomechanical Engineering 127:364-373, 2005
2. Anderson, A.E., Peters, C.L., Weiss, J.A. et al. Radiology, 246(1):133-41, 2008
3. Anderson, A.E., Weiss, J.A. et al. Proceedings, 8th Symposium of Computational Methods in Biomechanics
and Bioengineering. February, 2008
4. Anderson, A.E., Peters, C.L., Weiss, J.A. et al. Journal of Biomechanical Engineering, 130(5):051008, 2008