Patient Specific Modeling of Stage II Flatfoot Deformity Before

& After Surgery

EM Spratley, EA Matheis, RA Adelaar, CW Hayes, JS Wayne Richmond, Virginia

AOFAS Summer Meeting

June 20-23rd, 2012

Patient Specific Modeling of Stage II Flatfoot

Deformity Before & After Surgery

E. Meade Spratley, M.S.

My disclosure is in the Final AOFAS Program Book. I have a potential conflict with this presentation due to:

CW Hayes: “Board member/ committee appointments for a society” JS Wayne: “Other financial or material support from a company or supplier; “Board member/ committee appointments for a society” RS Adelaar: “Board member/ committee appointments for a society”

DISCLOSURES

Introduction • Adult Acquired Flatfoot Deformity (AAFD)

– Chronic & degenerative disease characterized by loss of normal arch function – Often secondary to PTT dysfunction – Causes static stretching of medial support structures of the arch, namely:

o spring, talocalcaneal interosseous, fibers of the anterior deltoid, & long /short plantar ligaments

– Presents clinically as a drop in medial longitudinal arch, forefoot abduction, & hindfoot valgus as well as significant pain & dysfunction

• Competing surgical treatments should be tailored to the individual patient’s

presentation

Objective Develop and validate a patient specific computational model to describe

foot/ankle function preoperatively and predict outcome of surgical correction

Study Design & Imaging • With IRB approval, lower leg & foot of a 64yo ♀ imaged with MRI & X-ray

– Pre-Op:

1. 1.5T MRI using a T2 SPGR fat-saturated sequence, yielding 0.7mm3-isovolumetric scan

2. Weight-bearing single-leg X-rays in the ML & oblique AP planes; Hindfoot photos

– Post-Op: 1. Weight-bearing single-leg X-rays in the ML & oblique AP planes; Hindfoot photos

• All bony tissue was isolated and extrapolated from the MRI scans, then triangulated to yield accurate patient-specific anatomy

Model Setup & Testing

Ligament of Interest Grade† Stiffness Attenuation

SuperoMedial Spring 1 25.0% InferoMedial Spring 0 0.0% Anterior Deltoid 1 25.0% Posterior Deltoid 0 0.0% Deep Deltoid 0 0.0% TaloCalcaneal Interosseous 0 0.0% Plantar Fascia 1 25.0%

• Full body weight axial load • All ligaments of the foot & ankle

included as tensile vectors • Muscle loading included as ratios of

body weight according to Thordarson et al.6 • Achilles = 50% BW • FHL = 10% BW • FDL = 6% BW

• Specific ligaments associated with AAFD were evaluated through MRI by CWH† & graded according to a scale of attenuation proposed by Deland et al1-3

.

BW 148lbs

Achilles 74lbs

FHL 15lbs

FDL 9lbs

Surgical Treatment & Post-Op Modeling

1. PTT augmentation by FHL transfer through bony tunnel in the navicular

2. 5mm Medializing Calcaneal Osteotomy

(MCO or ‘Slide’ Osteotomy) Fixed with cancellous screw

FHL

5mm

Validation Against Patient Radiographs • 6 angular measures used to

evaluate AAFD recorded for pre- & post-Op – ML (θ1- θ3)

1. calcaneal pitch (ML-CP) 2. talo-1st metatarsal (ML-T1MT) 3. talocalcaneal angle (ML-TC)

– Oblique AP (θ4, θ5) 4. talo-1st metatarsal angle (AP-T1MT) 5. talo-navicular coverage (AP-TN)

– Hindfoot (θ6) 6. PA hindfoot valgus (PA-HFV)

• Model compared to patient

data as well as AAFD population averages

– Coughlin & Kaz7 – Krans et al.8 – Murley et al.9

Results: Model to Patient Comparisons

• Pre-Op – All but 1 angle within 5º

• Post-Op – all measures within <5°

---------------------------------------

• Patient angles within 6.5º of

published AAFD averages7-9

Conclusions • A patient-specific flatfoot model was faithfully recreated in silica • Foot and ankle biomechanical function was dictated solely by:

– 3D articular anatomy – Ligaments – Muscle loading – & Body weight

• Model predictions of joint angles were very similar to patient radiographs & correctly predicted changes due to surgical treatment

Future Work • Future investigations can be used to assess changes in biomechanical

factors such as articular contact force / location or ligament strain as a means of predicting future patient outcome

• Additionally, these models could investigate new devices or surgeries in order to better tailor patient treatment

References

1. Deland JT, de Asla RJ, Sung IH, Ernberg LA, and Potter HG, 2005, “Posterior tibial tendon insufficiency: which ligaments are involved?,” Foot Ankle Int, 26(6), pp. 427-435.

2. Blackman AJ, Blevins JJ, Sangeorzan BJ, and Ledoux WR, 2009, “Cadaveric flatfoot model: Ligament attenuation and Achilles tendon overpull,” J Ortho Res, 27(12), pp. 1547-1554.

3. Kitaoka HB, Ahn TK, Luo ZP, and An KN, 1997, “Stability of the arch of the foot,” Foot Ankle Int, 18(10), pp. 644-648.

4. Iaquinto JM, and Wayne JS, 2011, “Effects of surgical correction for the treatment of adult acquired flatfoot deformity: A computational investigation,” J Ortho Res, 29(7), pp. 1047-1054.

5. Spratley EM, and Wayne JS, 2010, “Computational Model of the Human Elbow and Forearm: Application to Complex Varus Instability,” Ann Biomed Eng, 39(3), pp. 1084-1091.

6. Thordarson DB, Schmotzer H, Chon J, and Peters J, 1995, “Dynamic support of the human longitudinal arch. A biomechanical evaluation,” Clin Orthop Relat Res, (316), pp. 165-172.

7. Coughlin MJ, and Kaz A, 2009, “Correlation of Harris mats, physical exam, pictures, and radiographic measurements in adult flatfoot deformity,” Foot Ankle Int, 30(7), pp. 604-612.

8. Krans A van der, Louwerens JWK, and Anderson P, 2006, “Adult acquired flexible flatfoot, treated by calcaneo-cuboid distraction arthrodesis, posterior tibial tendon augmentation, and percutaneous Achilles tendon lengthening: A prospective outcome study of 20 patients,” Acta Orthop, 77(1), pp. 156-163.

9. Murley GS, Menz HB, and Landorf KB, 2009, “A protocol for classifying normal- and flat-arched foot posture for research studies using clinical and radiographic measurements,” J Foot Ankle Res, 2, p. 22.

10. Thomas J, Kunkel M, Lopez R, and Sparks D, 2006, “Radiographic Values of the Adult Foot in a Standardized Population,” J Foot Ankle Surg, 45, pp. 3-12.