Adult Acquired Flatfoot Deformity: Biomechanics and Effective Treatment

Kevin A. Kirby, DPMClinical Associate Professor

Department of Applied Biomechanics California School of Podiatric Medicine

Oakland, California, USA

Tibialis Posterior Dysfunction

� Described in 1991 by Mueller as pathology with multiple etiologies affecting the posterior tibial (PT) tendon

� Can be a seriously disabling condition causing pain, weakness and progressive development of flatfoot deformity

Mueller TJ: Acquired flatfoot secondary to tibialis posterior dysfunction: biomechanical aspects. J. Foot Surgery, 30:2-11, 1991.

� Also known as “tibialis posterior dysfunction”, “posterior tibial dysfunction” and “adult acquired flatfoot deformity”

PTTD Staging

� Multiple staging systems proposed by various authors to classify the clinical severity of PTTD

� Most commonly used staging system is that proposed by Johnson and Strom in 1989 that was later modified by Myerson in 1996

Johnson KA, Strom DE: Tibialis posterior tendon dysfunction. Clinical Ortho., 239:196-206, 1989.

Myerson MS: Adult acquired flatfoot deformity: treatment of dysfunction of the posterior tibial tendon. JBJS, 78:780-792, 1996.

Functions of the Posterior Tibial Muscle

� PT muscle functions to cause:-ankle joint plantarflexion moment-STJ supination moment-MTJ adduction and plantarflexion moment

� PT tendon passes posterior to ankle joint axis, medial to STJ axis, and medial-plantar to midtarsal joint (MTJ) axis

Functions of the Posterior Tibial Muscle

� During first half of stance phase of walking gait, primary function of PT muscle is to:

� Decelerate STJ pronation� Decelerate forefoot abduction at MTJ� Decelerate forefoot dorsiflexion at MTJ

Function and Dysfunction of the Posterior Tibial Muscle

� During latter half of stance phase, primary function of PT muscle is to:

� Accelerate STJ supination� Accelerate forefoot adduction at MTJ� Accelerate forefoot plantarflexion at MTJ� Loss of PT function results in large

decrease in STJ supination moments, forefoot adduction moments and forefoot plantarflexion moments

Biomechanical Significance of STJ Axis Location in PTTD

� Research with Penn State Biomechanics Lab involves using motion-based method to determine STJ axis location in both cadaver and live subjects without drilling pins into talus

Lewis GS, Kirby KA, Piazza SJ: Determination of subtalar joint axis location by restriction of talocrural joint motion. Gait and Posture. 25:63-69, 2007.

Isolating STJ Axis in Live Subjects by Motion Restriction of Ankle Joint

� Fast MRI scans performed on 4 subjects with special apparatus that rotates STJ while limiting ankle joint motion

� STJ axis determined from tibia-calcaneal motion averaged only 6.00 and 2.5 mm from “true” STJ axis determined by MRI scans

Lewis GS, Cohen TL, Seisler AR, Kirby KA, Sheehan FT, Piazza SJ: In vivo tests of an improved method for functional location of the subtalar joint axis. J Biomechanics, 42:146-151, 2009.

STJ Axis Palpation Technique

Kirby KA: Methods for determination of positional variations in the subtalar joint axis. JAPMA, 77: 228-234, 1987.

STJ Axis Determination on Feet with Normal STJ Axis Location

Points of No Rotation

Results of STJ Axis Determination Led to Classification System

� Feet which functioned most normally had STJ axes which passed from posterior-lateral calcaneus to first intermetatarsal space

� Feet which had pronation-related symptoms and pronation gait abnormalities had “medially deviated STJ axes”

� Feet which had supination-related symptoms and supination gait abnormalities had “laterally deviated STJ axes”

Mechanical Effects of Medial and Lateral Deviation of STJ Axis

� Medial and lateral deviation of STJ axis affects direction and magnitude of rotational forces (moments) acting across STJ axis

� STJ deviation alters effects of ground reaction force (GRF), muscular contractile forces, and ligamentous tensile forces during weightbearing activities

Alteration of Rotational Effects of GRF with STJ Axis Deviation

STJ Axis Deviation Alters Muscular Moments on STJ in PTTD

Severely Medially Deviated STJ Axis in Patient with PTTD (Right Foot)

STJ Axis Location is More Medially Translated and Adducted in PTTD

STJ Axis Medial Deviation Greatly Affects PT Muscle Function

� Increased pronation motion of STJ will cause internal rotation and medial translation of STJ axis in relation to PT tendon

� As STJ axis becomes more medially deviated, PT tendon moment arm to STJ axis is shorter

� Results in PT muscle having reduced capacity to generate important STJ supination moment during weightbearing activities

� Shortened PT tendon supination moment arm creates need for increased PT muscle contractile force to produce a given magnitude of STJ supination moment

� Increased tensile stress on PT tendon results that may cause tendon injury of PT tendinitis/PTTD

Kirby KA: Conservative treatment of posterior tibial dysfunction, Podiatry Management, Vol 19, No 7, pp. 73-82, 2000.

STJ Axis

Posterior TibialTendon

Normal STJ Axis Position Medially Deviated STJ Axis

STJ Axis

Normal length STJ axis-PT moment arm

Shortened STJ axis -PT moment arm

Typical Stress-Strain Curve for Ligament and Tendon

Strain

Stre

ss

Toe Region

Linear Region

ProportionalLimit

ElasticLimit

Yield Point

Ultimate Stress

Rupture Point

Region of Optimum TendonStress

Region of PathologicalTendon Stress

Loss of Tendon Diameter Greatly Increases Tensile Stress on Tendon� Tensile stress = tensile force

per cross-sectional area (F/A), in pascals (Pa) = N/m2

� When PT tendon develops a tear, reducing cross-sectional area, tensile stress in tendon increases in area of tear

� Increased tensile stress in tendon fibers increases risk of further tear or rupture

Forefoot GRF and STJ Axis Location

� Increased STJ axis medial deviation also increases medial distance from forefoot to STJ axis which increases STJ pronation moment from GRF

� STJ pronation moments are, therefore, greatest in late midstance when forefoot GRF is greatest

Effects of Decreased PT Muscle Moment Arm Across STJ Axis

� Causes decreased STJ supination moment for given magnitude of PT tendon force

� Increases PT tendon force required to produce given magnitude of STJ supination moment

� PT muscle becomes unable to generate sufficient STJ supination moment to decelerate STJ pronation in early stance and accelerate STJ supination in late stance phase

� Result: increased STJ pronation motion

Increased PT Tendon Tensile Force Leads to Tendon Pathology� Chronic high tension on PT tendon increase chances

of tendon inflammation, elongation or tearing � Any inability of PT tendon to transmit tensile forces

may lead to PTTD since STJ pronation moments are not counterbalanced by STJ supination moments

http://www.footankleinstitute.com/TendonInjury.html

Increased STJ Pronation Moments Lead to Ligament Pathology

� Spring ligament complex may also pathologically elongate or may develop structural defects due to excessive STJ pronation moments seen in PTTD

� Spring ligament complex consists of superomedial calcaneonavicular (SMCN) and inferior calcaneonavicular (ICN) ligaments

Davis WH, Sobel M, DiCarlo EF, et al: Gross, histological, microvascular anatomy and biomechanical testing of the spring ligament complex. Foot Ankle Int. 17:95-102, 1996.

Spring Ligament Complex� Superomedial calcaneonavicular (SMCN) ligament� Inferior calcaneonavicular (ICN) ligament

Spring Ligament Elongation Leads to Increased Deformity

� If posterior tibial tendon can not develop adequate tension to intermittently reduce tensile forces on spring ligament complex, then eventual plastic deformation, elongation or failure of ligaments may occur

Spring Ligament Elongation Leads to Increased Deformity

� Elongation or failure of spring ligament complex leads to further medial longitudinal arch collapse

� Spring ligament failure also leads to further abduction of forefoot on rearfoot

Spring Ligament Elongation Leads to Increased STJ Pronation Moments

� Increased abduction of forefoot increases STJ pronation moment arm for GRF (i.e. CoP) which, further increases STJ pronation moments in late midstance

Clinical Appearance of PTTD

Increased Medial Convexity Inferior to Medial Malleolus

Clinical Appearance of PTTD

Increased Convexity Medial Midfoot Border

Talar Bulge Medially Positioned and Internally Rotated

STJ Axis Determination in PTTD

Severely Medially Deviated STJ Axis

Modified Muscle Testing for PTTD

Dorsiflexion –abduction force on plantar-medial first metatarsal head

Palpation for PT tendon tension and integrity

Lateral ankle braced against leg of examiner

Goals for Treatment of Posterior Tibial Tendon Dysfunction

� Reduce STJ pronation moments acting on foot by increasing STJ supination moments with orthoses and shoegear

� Reduce inflammation, edema and pain in PT tendon to allow more normal PT muscle function and more normal gait function

� Increase strength of PT muscle after inflammation has been reduced to improve ability of PT muscle to generate STJ supination moments

Typical Foot Orthoses for Mechanical Treatment of PTTD

� 3/16 - 4/16” polypropylene shell� 40/40 rearfoot posts� 3 mm to 6 mm medial heel skive� Heel contact points 1/8” – 3/16” thick� 18 mm to 20 mm heel cup height� Minimal medial expansion on positive cast � Positive cast balanced 20 - 60 inverted

Use of Medial Heel Skive in Treatment of PTTD

� Medial heel skive technique is orthosis modification which allows for precise amounts of varus wedging to be added into heel cup of orthosis in order to increase STJ supination moments acting on foot during weightbearing activities

Kirby KA: The medial heel skive technique: improving pronation control in foot orthoses, JAPMA, 82: 177-188, 1992.

Medial Heel Skive:Cast Construction Technique

Feet with PTTD Have Reduced Plantar Area Medial to STJ Axis

� As STJ axis becomes medially deviated, foot orthosis has decreased surface area medial to STJ axis by which to generate STJ supination moments

Medially Deviated STJ Axis of PTTD Affects Function of Orthosis

� Foot orthosis needs to have more “pronation control” features as STJ axis becomes more medially deviated in PTTD

� Medial arch of foot may not be medial to STJ axis in foot with severely medially deviated STJ axis

� Medial calcaneus is area of plantar foot with longest supination moment arm to STJ axis

� Medial calcaneus is, therefore, a very important area on plantar foot for foot orthosis to generate STJ supination moments in PTTD

Synergy of Medial Heel Skive and Increased Orthosis Medial Arch Height

� Medial heel skive without increased medial arch height causes too little medial longitudinal arch force from orthosis to optimize pronation control

� Increased medial arch height without medial heel skive causes excessive medial arch force from orthosis

� Combination of both medial heel skive and increased medial longitudinal arch height in orthosis will result in optimum synergistic STJ supination effect from orthosis

PTTD Orthoses Shift Reaction Forces More Medially on Plantar Foot

PTTD,Shoes Only

PTTD,Orthoses in Shoes

F-Scan Images Courtesy of

Bruce Williams, DPM

Areas of increased medial arch and medial heel pressure from medial heel skive orthosis

High Top Boots/Shoes are Needed Along With Foot Orthoses for PTTD

� Very important that patient wear orthoses in high top hiking boots or high top shoes to increase orthosis efficacy

Biomechanical Effect of High Top Boots in PTTD

� High top boot/brace acts synergistically with medial heel skive orthosis to increase STJ supination moment

� Boot acts both superior and inferior to STJ axis to exert STJ supination moment

� Foot orthosis can only act inferior to STJ axis

Clinical Effect of Orthoses and Boots on PTTD

� High top boots and foot orthoses work together to realign STJ axis to a more normal position

Alternatives to High Top Boots and Foot Orthoses for PTTD

� Arizona Ankle Brace� Southwest Ankle

Brace� Platinum Ankle

Brace (PAL)� ProLab Functional

AFO� Richie Brace

Ankle-Foot Orthoses in PTTD

� Double steel upright hinged ankle brace with medial T-strap is also very effective at treating PTTD

STJ Axis

STJ Axis

MedialT-Strap

STJ Supination Moment

STJ Axis

MedialT-Strap

STJSupinationMoment

Important to Reduce Local Inflammation in PTTD

� Patients are started on twice daily icing therapy (20 minutes/session) in order to reduce local inflammation in PT tendon

� Icing is continued until medial ankle edema and pain has diminished

� NSAIDS may also be helpful at reducing the pain and edema at medial ankle area

� Less swelling around tendon leads to better tendon function

Strengthening of PosteriorTibial Muscle

� Patients are started on strengthening exercises for PT muscle once they have worn orthoses for three weeks and/or tendon has become less symptomatic

� Exercises can consist of isometric exercises or concentric and eccentric adduction exercises with tire inner-tube or theraband

Biomechanics of Surgical Treatment of PTTD

� Any displacement osteotomy of calcaneus or midfoot may affect STJ axis location and mechanical function of foot and lower extremity

� Medial displacement osteotomies of calcaneus increase supination moment arm for GRF acting on plantar calcaneus and also increase supination moment arm for Achilles tendon

� Mechanical effects of frontal and transverse plane calcaneal and midfoot osteotomies are easily understood using STJ axis location/rotational equilibrium theory

Kirby KA: Subtalar joint axis location and rotational equilibrium theory of foot function. JAPMA, 91:465-488, 2001.

� 1. Posterior calcaneal displacement osteotomy causes medial shift in plantar calcaneus relative to STJ axis which increases STJ supination moment when GRF acts plantar to calcaneus

� 2. Posterior calcaneal displacement osteotomy also causes a medial shift in Achilles tendon force relative to STJ axis which increases supination moment during late midstance and propulsion, when Achilles tendon tensile forces are greatest

Intraoperative Assessment of Calcaneal Osteotomy with STJ Axis Palpation

� STJ axis palpation technique used during calcaneal displacement surgery to assess for optimum calcaneal alignment

Roukis TS, Kirby KA: A simple intraoperative technique to accurately align the rearfoot complex. JAPMA, 95:505-507, 2005.

Anterior Axial Projection of Foot

� New radiographic projection, Anterior Axial Projection, invented in 1985

Kirby KA, Loendorf AJ, Gregorio R: Anterior axial projection of the foot. JAPMA, 78: 159-170, 1988.

� Anterior axial projection allows reproducible method of obtaining frontal plane image of relative positions of plantar rearfoot, forefoot and ankle mortise while the patient stands in angle and base of gait

Anterior Axial Positioning Device� Made of radiolucent

plastic with longitudinal bisection line on top

� Has steel pin embedded within posterior edge of top surface to act as horizontal reference line

Photo courtesy, J. Gerbert, DPM

Clark JR, Gerbert J, Jenkin WM: The Kirby view: A radiographic view for flatfoot evaluation. JFAS, 43:436-439, 2004.

Characteristics of Anterior Axial Projection of Foot

� Central beam inclinated 100 from anterior to posterior with x-ray tube below level of foot

� X-ray plate (i.e. film) positioned vertically and against posterior aspect of heel of foot

Central beam

X-ray plate

X-ray tubePositioning devicePhoto courtesy of J. Gerbert, DPM

� Patient is positioned on device with center of second digit and center of plantar calcaneus resting on longitudinal bisection line

� Patient is positioned in angle and base of gait with opposite foot resting on surface of same height as positioning device

Longitudinal bisection line

� Anterior axial projection readily demonstrates plantar contours of calcaneus and its position relative to ankle mortise

� Plantar contours of medial and lateral calcaneal tubercles are well visualized relative to horizontal reference marker and relative to ankle mortise

Horizontal reference marker

Normal

Pes Planus Pes Cavus

� Anterior axial projection is used to document relative changes in calcaneal position with posterior displacement calcaneal osteotomies which are commonly used for surgical treatment of PTTD

Right foot, post-op

Left foot, pre-op

Surgical photos courtesy of Joshua Gerbert, DPM

Posterior Displacement Osteotomies of Calcaneus� Preoperatively, plantar calcaneus is laterally

positioned relative to ankle mortise� Postoperatively, plantar calcaneus is more

centrally located relative to ankle mortise

Summary of Treatment Approaches for PTTD

� PTTD may be successfully treated both non-surgically and surgically

� Clinical decision of when to treat PTTD surgically should be based on patient response to conservative care and level of activity

� Understanding the biomechanics and pathophysiology of PTTD is critical to developing an optimal treatment plan for patients with this painful and potentially disabling disorder

Thank you!

Thank you!