BIOMECHANICS OF FAR CORTICAL LOCKINGMichael Bottlang & Florian FeistJournal of Orthopaedic trauma ,Feb 2011.Presentation by: RAM SUDHAN.S.

NEED FOR FAR CORTEX LOCKING SCREW:

COMPLICATIONS OF LCP’s:

SUPRACONDYLAR # FEMUR:Non union rate – 18 – 23 %

BOULTON C et al (AAOS 2011 )– 23%HOFFMANN et al (J ortho surg Res

2013) – 18 % , incl’ 10% - hard ware failure.

STANDARD LCP : (bridge plating) depends on secondary bone healing rather than primary bone

healing

traditionally achieved by external fixation constructs

more flexible inter fragmentary motion ( 10 times more than LCP / DCP)

inherently produce rigid stabilisation - kubiak et al 2006 (JBJS Am)

can supresses interfragmentary motion ( HIGH STIFFNESS) BOTTLANG et al 2010 (JBJS Am), lujan et al 2010 ( J orthop trauma)

insufficient for secondary # healing.

1. rigid locking screws - uneven stress distribution. stress shielding under the plate.

bone resorption

To enable flexible fixation with LCP’s / to reduce the stiffness

FAR CORTEX LOCKING SCREWS WERE DEVELOPED

FAR CORTEX LOCKING: WHAT IT HAS: ( 4 KEY FUTURES)

1.Flexible fixation. ( reduces stiffness by 80- 88%)

actively promotes callus formation similar to external fixator.2.Uniform load distribution – mitigate stress raisers.3.Progressive stiffening – on increased loading.4.Parallel inter-fragmentary motion – by ‘S’ shaped flexion.

[ FUNCTIONS AS EXTERNAL FIXATORS IN BIOMECHANICAL BEHAVIOUR AND BIOLOGIC HEALING ]

FLEXIBLE FIXATION : Flexibility: directly proportional - working length of the screw.

Stiffness is reduced by screws – fixed in plate & far cortex, while retaining a controlled motion envelope at near cortex.

thereby promoting interfragmentary motion

[ IDEAL: 0.2 – 1 mm ]

to produce secondary callus

PARTS:1. HEAD – locking thread2. MOTION CONTROL COLLAR3. FLEXIBLE SHAFT reduced diameter elastically deflect ( within - motion control collar of near cortex)4. FAR CORTEX LOCKING THREADS

resembling an monolateral external fixator

FCL CONSTRUCT : reduces the stiffness of, FEMORAL DIAPHYSIS – 88% , FEMORAL METAPHYSIS – 80% TIBIAL DIAPHYSIS – 84% [ Bottlang et al 2009,2010 JBJS Am ]

COMPARISON OF STIFFNESS: LCP- FCL – EX’FIX

the FCL lowers - axial stiffness by 84% than standard LCP construct (4286N/mm)

FCL --- APPROX’ EQUAL --- EXTERNAL FIXATOR(682N/mm) (488N/mm)

produces inter fragmentary motion (0.6mm)

Suitable for promoting callus formations

LOAD DISTRIBUTION: IN LCP: Transmits load thro’ fixed angle screws induce stress concentration at screw bone interface uneven load distribution esp: outermost locking screw (prox/dist) stress shielding in adjacent regions increases the # risk at the plate end 1. cortical porosis or 2. delayed bridging.

FOCUSSED LOAD TRANSFER

IN FCL: strain

Distributed equally to entire working length – all screws.

supresses stress raisers at ends of plate

Suitable in healthy boneOsteoporotic Diaphysis stress shielding & porosis

focussed

Adjacent to near cortex LCP Screw segment b/w near & far cortex

Remains latent

PROGRESSIVE STIFFENING: ( similar- ilizorov fixator) SHOWS BIPHASIC STIFFNESS

Normal loading elevated loading

Low initial stiffness elastic flexion gains addition support at near cortex(All load – plate to far cortex) ( 6 fold increase in construct stiffness )

Thro’ flexible shaft SIMILAR TO LCP

promotes inter fragmentary motion protects the bone from( in early healing phase) excessive motion / loading

HOW IT DOES THAT: ( motion control collar) FCL screw DIA’ at near cortex is sufficiently large

To confine the flexion within its elastic range

Thus excessive flexion is prevented

Thereby , screw fatigue – prevented.

MIMICKING THE FCL SCREW FUNCTION:

overdrilling the near cortex with standard locking screws by 1 mm

overloads the far cortex

due to lack of flexibility of standard screw

PARALLEL INTERFRAGMENTARY MOTION: ON AXIAL LOADING

Bridge plating constructs – plate flexion ( elastic plate bending)

Coz’ plate – offset from cortex.

Enables inter fragmentary motion

BUT,

LCP

INCREASING MOTION ON FAR CORTEX

DUE TO, RIGID FIXATION – NEAR CORTEX

ASYMMETRIC GAP CLOSURE

ASYMMETRIC CALLUS FORMATION[ LUJAN et al 2010 – J orthop trauma ]

FCL

PARALLEL interfragmentary motion

Due to flexible shaft produces – ‘S’ SHAPED FLEXION

THUS, symmetric callus – formed.

FCL:1. SYMMETRIC

HEALING

2. INCRESED BONE MINERAL DENSITY BY 44%

3. TOLERATES 156% MORE ENERGY TO FAILURE.

CONSTRUCT STRENGTH:LCP FCL

AXIAL COMPRESSION 7% LOWER

BENDING 24% HIGHER

TORSION 54% HIGHER

CONCLUSION: FCL CONSTRUCT PROVIDES:

FLEXIBILITY UNIFORM LOAD PROGRESSIVE PARALLEL DISTRIBUTION STIFFENING INTER FRAGMENTARY MOTION

PROMOTES CALLUS FORMATION - PREVENTING PARTIAL NON UNION

RESEMBLES internal fixation by construct but replicates the biomechanical function of external fixators.

THANK YOU…