Licensee OA Publishing London 2013. Creative Commons Attribution License (CC-BY)

Patel V, Venkatarayappa I, Prayson MJ, Goswami T. Biomechanical evaluation of hybrid locking plate constructs. Hard Tissue 2013 Jun 01;2(4):32.

Com

petin

g in

tere

sts &

Con

flict

of i

nter

ests

: Non

e de

clar

ed.

All a

utho

rs c

ontr

ibut

ed to

con

cept

ion

and

desig

n, m

anus

crip

t pre

para

tion,

read

and

app

rove

d th

e fin

al m

anus

crip

t.

All a

utho

rs a

bide

by

the

Asso

ciat

ion

for M

edic

al E

thic

s (AM

E) e

thic

al ru

les o

f disc

losu

re.

1

Biomechanical evaluation of hybrid locking plate constructs

V Patel1, I Venkatarayappa2, MJ Prayson2, T Goswami1, 2

1. Department of Biomedical, Industrial and Human Factors Engineering

2. Orthopaedic Surgery, Sports Medicine and Rehabilitation

Wright State University, 3640 Colonel Glenn Hwy, Dayton, OH 45435

Abstract

Hybrid locking compression plate constructs is commonly used in fracture stabilization

of long bones. Locking compression plates may be used with locking and non-locking

screws. Locking screws have the threaded heads, which enable to lock the screw within

the plate hole and eliminate motion between screw and plate. They provide stability to

the fixation and are most suited for osteoporotic bones and in stabilization of fracture

with bone defects and significant comminution. Non-locking screws are suitable for

normal bones and for simple fractures.

Recommendation for the optimal combination of locking and non-locking screws is

desired in an operating room setting though limited information available in the literature.

Cost of locking screw is up to ten times more than non-locking screw. In order to reduce

operating room costs, there is a need to determine an optimum arrangement of locking

and non-locking screws that provides the best dynamic stiffness and stability of the

construct during and after the healing occurred. Fracture models with synthetic femora

were created using locking compression plate and different combination of locking and

non-locking screws. Four groups with five samples in each group were tested for axial

and rotational stiffness. Simulated solid models were created using Solidworks modeling

2

and finite element analysis was performed and the results were compared with

experimental results. The results from this experimental program indicate that locking

screws should be placed near the fracture gap to gain axial and torsional strength. Also,

locking screws impart higher resistance to loosening of the adjacent level non-locking

screws as found in this synthetic femur study.

Keywords locking compression plate, locking screw, non-locking screw, dynamic

stiffness, finite element model, displacement

3

1.0 Introduction

An open reduction and internal fixation (ORIF) procedure is performed over 4 million

times in the United States annually. A hybrid plate construct involves the use of locking

screw and conventional cortical or cancellous screw. Internal fixation devices work as

load sharing or load bearing devices. Plate osteosynthesis in comminuted fracture acts as

load bearing device where the plate assumes all the loads. Compression plating in simple

fracture pattern works as load sharing devices where bone assumes most of the load.

Fracture healing leading to bone union depends on the devices used and their

biomechanical behavior. Hence it is important to obtain stable fixation to prevent implant

failure. Implants are not routinely removed unless there is infection or mechanical

failures. Internal fixation devices are of many types. Some of the commonly used

devices include K-wires, Plates, Screws, intramedullary rods etc., K-wires are mainly

used as reduction tool and sometimes in stabilizing fractures of small bone in hand and

feet.

Different types of plates used for internal fixation include locking compression plate,

dynamic compression plate, and reconstruction plate. Dynamic and locking compression

plates are more commonly used. Locking compression plate acts as internal external

fixator. They have very limited contact with the bone and hence reduce vascular damage.

This has a great advantage in osteoporotic bone and in fractures with significant

comminution and bone loss. Locking compression plates provide relative stability at the

fracture site and facilitate bone healing by secondary intention1-10.

Conventional plate osteosynthesis offers stability by friction between plate-bone

interface. On axial loading, bending force is applied on the screws, which generate a

4

shear force. When axial load exceeds the friction force, movement occurs between the

plate and screws, known as toggle, and may result in bending or fracture of screws. The

change in plate screw angle limits failure loads to the strength of thread purchase.

Screws alone may be used as primary mode of fixation in small bone fractures in hands

and feet. Intramedullary rods are more commonly used in midshaft fractures of femur

and tibia.

Previous studies have evaluated the mechanical properties of hybrid plate construct in fourth-generation composite bone model11. The potential advantage of locked screws in improving the axial and torsional strength and different combination of locked and non- locked screws have been studied12. To our knowledge there is no study, evaluating the dynamic stiffness of Hybrid construct at different stages of healing process.

2.0 Materials and Methods

In this study synthetic femur models (Model 3403, pacific Research Laboratories,

Vashon, USA) which contain epoxy glass fiber as shallow cylinders filled with

polyethylene were used. Fracture comminution was simulated by an osteotomy gap of

2cm in the metaphyseal region using twenty synthetic femur models. Ten hole pre-

contoured lateral distal femoral locking plate implants were used and simulated fractures

were stabilized with different combination of locking and non-locking screws. The femur

fracture constructs were divided into four groups as shown in Table-1. Five femurs were

prepared for each of the 4 groups. The test protocol was kept same for all 4 groups of

testing where load was cycled in the negative Y direction from 50 to 700 N and a torsion

rotation of + 5º /cycle applied in a sinusoidal waveform. At the end of 50,000 cycles tests

were stopped. The details of the specimen preparation and specimen testing are available

5

elsewhere10-11. Simulated solid models were constructed to perform finite element

analysis so that these results could be compared with the experimental results.

2.1 Solidworks Modeling

Solid models of LCP were created similar to Synthes condylar plates through Solidworks

2007, a three-dimensional (3D) Computer Aided Design (CAD) software. The non-

locking screw heads were minimized in size to decrease contact with the plate hole as

shown in Figure 1. The distal condylar flared portion of the plate was designed with loft

and fillet (Loft and Fillet are features used in Solidworks for creating transitions and

external/internal round surfaces, respectively) operations as shown in Figure 2. A final

assembly was created joining the various individual model portions. This assembly was

similar in appearance to the femoral constructs tested experimentally. Additional groups

were created for completeness. Plating constructs with three locking screws and one non-

locking screw were developed as well as plates with four locking screws. The bony

segments (condylar and shaft) were subtracted for further analysis in ANSYS, (ANSYS

Inc, Canonsburg, PA). Appropriate constraints were applied at the plate ends and on the

screws. The model was then saved in a para-solid format. Three-dimensional models of

the femur were also created using Mimics (Materialise, Inc, Plymouth, MI), which is a

3D modeling software that imports computed tomography (CT) imaging data. The CT

images of the specimens were acquired from Miami Valley Hospital in Dayton, OH.

Three-dimensional models, similar to the anatomical specimens, were achieved using the

Mimics tools as seen in Figure 3.

6

2.2 FEA analysis in ANSYS

The Solidworks models stored in parasolid format were imported into ANSYS. Stainless

steel (316L) properties were assigned to the plate and screws built with SHELL 93

elements. Loads were applied to the screws with the plate and screws constrained. A

time harmonic mode was chosen which converts loads in terms of displacement and

stress. Loads of 300N were applied to each screw in a negative Y direction (downward).

From the obtained results, construct dynamic stiffness and deformation were measured.

3.0 RESULTS

3.1 Finite Element Analysis

Table-2 summarizes the results of finite element analysis in terms of maximum stress and

maximum displacement that occurred in different screws. Maximum stress occurred in

plates with all non-locking screws. Stresses were the lowest in plates with all locking

screws. A maximum stress of 449 N/m2 occurred in plates with all non-locking screws,

427 N/m2 in plates with three non-locking screws, 373 N/m2 and 341 N/m2 in plates with

two and one non-locking screws, respectively. A lowest maximum stress of 190 N/m2

occurred in plates with all 4 locking screws. Total displacement of locked construct was,

however, higher than all other combinations. It may be noted that the displacement

considered here were in nano-meters, and operating room practice allows 10% of the

fracture gap as a rule of thumb. Figure 4 shows the ANSYS models with all 4 locked and

non-locked constructs.

3.2 Experimental Program

Experimental data generated on 20 synthetic femurs has been tabulated in Table-3.

Groups 2 and 3 exhibited higher torque to loosen the screws. However, locking screws

7

when positioned either immediately after the osteotomy gap, or at the distal end of the

plate, exhibited similar amount of torque required to loosen the screw. More than one-

half of torque was retained in the screw at the end of 50,000 cycles. Constructs made with

conventional screw, on the other hand, loosened most of their insertion torque and were

loose enough that torque meter did not read the remaining torque. Remaining torque was

measured by subtracting the loosening torque from insertion torque.

The conventional screw, located in Screw-2 position, maintained somewhat lower torque

than Screw-3. The average torque of locking screws, farthest and closest from the

osteotomy gap, remained very similar10-11. Similarly, non-locking screws farthest from

the gap showed low loosening torque compared to the non-locking screws nearer to the

fracture gap. Table-4 summarizes average remaining torque, percentage loosening torque,

axial stiffness, average torsional stiffness, and average deformation of each of the screws.

It may be noted that a construct, collectively with locking and non-locking screws, had an

axial stiffness range from 200 to 4000 N/mm, whereas the torsional stiffness range was

50-350 N×mm/º rotation and average deformation range was 0.5-2.5 mm.

The dynamic stiffness was plotted with respect to the number of cycles for all the 4

groups, Figures 6-9. The dynamic stiffness in Group 1 varied with respect to number of

cycles in a sine function, representative of harmonic oscillation and may indicate the

density and/or porosity variation in the construct, and is not of interest in this study.

However, the Group 1 harmonics may indicate severe displacement changes during the

tests. The displacement data was reviewed for all the groups and no trends were found

that can be reported here. In all the tests, the displacement at the end of 50,000 cycles was

the highest. The total deformation varied, as noted earlier from 0.5 to a worst case

8

scenario of 2.5 mm. The displacement data for Group 1 is shown in Figure10 showing

normal, gradual accruing of deformation as the cycles continue. Since the significant drop

in stiffness occurs during the initial stages of cyclic loading, as shown in Figures 6-9, the

first 20,000 cycles are therefore very important in fracture fixation and healing. The

remaining torque begins to deplete at this point and may be the reason why loosening of

screws or screw pull-out is observed as a failure mode. The negative slope of the dynamic

stiffness for Group 4 constructs may be described by

Dynamic stiffness = -1E-20N5 + 2E-15N4 - 1E-10N3 + 3E-06N2 - 0.0403N+ 503.23

Where, N is the number of cycles.

The efficacy of a hybrid construct depends on the dynamic stiffness exhibited by the

construct. From Figures 6-9 it is obvious that lower stiffness ranges found for Groups 1-3

may loosen the constructs in the long run, and must show the remaining stiffness higher

than 100-200N/mm. Therefore, Group 4 provides the most effective fixation stability

with 2 locking screws one on each proximal and distal end from the osteotomy. Fixation

efficacy was found to be with the use of 2 locking screws for the study groups

investigated in this paper with dynamic stiffness range of 200 N/mm at the end of 50,000

cycles. However, the dynamic stiffness was expected to be much higher during the

insertion of the device, 500 N/mm, as shown in Figure 9 using 2 locking screws. None of

the other constructs show higher starting stiffness as Group 4. The starting stiffness

ranges from 100 to 300 N/mm. This study used 2 cm osteotomy gap, where high

stress/strain was likely to develop causing significant amount of axial and angular

displacements. Additional work is recommended for smaller osteotomy gaps to determine

the efficacy of the hybrid locking plate constructs.

9

4. DISCUSSION

The LCP is subjected to static and cyclic loads in vivo which generate extremely

complicated stress systems in the device2. Materials with a very high Young’s Modulus

make the plate stiff. Stiff plates do not transfer as much stress to the bone in the local area

causing the bone to thin or deplete its composition. This phenomenon is known as stress

shielding and causes osteopenia3. It is usually visible on the X-rays showing a region

darker than the superior or inferior regions. Therefore, materials that possess suitable

stiffness must be considered for use as LCP. Materials with a low modulus of elasticity

do not provide enough rigidity to the bone to heal the fracture, and materials with a high

elastic modulus increase rigidity and lower stresses in the bone; thus causing stress

shielding. For the fixation devices, stainless steel- 316L, remains the preferred material of

choice and used in this study.

Efficacy of the LCP performance depends on fracture type. Conventional compression

plate performs well for normal quality of bone and fracture with normal or partial contact

between fragments. When both ends of the bone fragments are not in contact with each

other, a bridge plate technique may be used either with locked or standard screws based

on the bone quality. A combination technique is employed for simple oblique or articular

fracture with more standard screws and fewer locking screws. An osteotomy gap of 2 cm

was used in this study, simulating significant bone loss and fragmented fracture. The

length of LCP is dependent on fracture length and loads being transferred to the plate

(e.g. bending, pull out)4. The ratio of the plate length to the fracture length is called plate

span width5. Guatier and Sommer recommended plate span width to be from 2 to 3 for

comminuted fracture and 8 to 10 for simple fracture6. This suggests that for more

10

comminuted fracture; a long plate provides higher axial and torsional stability than the

short plate7. Working length is the length between two screws of two different fracture

fragments. With a small fracture length and working length the bone ends do not come in

contact with each other thus reducing callus formation8. This increases stress and strain

during torsion loading. Stresses induced in a plate with a 6 mm fracture gap are higher

than plate with a 1 mm fracture gap. The stress in a screw decreases with smaller fracture

gap7. In osteoporotic bone, the working length is kept small because of small bone

thickness compared to the normal bones. When torsion load is applied, the screws tend to

pullout.

The LCP plates have point contact at the undersurface thus preserving periosteum blood

supply. Conventional plates exert 2000-3000N force when screws are tightened to the

bone [9]. LCP plates reduce this load and preserve the blood supply. Reduced contact

between the bone and the plate improves bone growth9. Stoffel suggested that by

increasing distance of plate from the bone 2 to 6 mm, the axial and torsional stability

decreased by 10-15%7.

Freeman et al. have studied the effect of the number and location of locked screws on the

mechanical properties of hybrid plate construction on osteoporotic bone model. Seven

different constructs with 2 unlocked and 5 hybrid constructs were tested. They

determined that 4 screws per side increased the stiffness to 33% when compared to 3

screws in each fragment. They did not find any difference when 1 or 2 unlocked screws

were replaced by locked screws on each fragment. However, replacing 3 unlocked screws

on each side with locked screws increased the torsional stiffness by 24%. They concluded

that at least 3 bicortical locked screws on each side of the fracture are needed in a

11

osteoporotic bone model to increase the torsional strength. Locked screws when placed

between the fracture and unlocked screws protect the unlocked screws from loosening12.

While literature cited effects of these parameters on resulting stiffness and stability of the

fixation, limited amount of data was available showing the dynamic stiffness of the

construct. The dynamic stability is pivotal in holding the construct especially in early

phase during fracture healing. Therefore, this paper provides the most desired dynamic

stiffness of hybrid constructs with different screw configurations at various stages of

healing process that will be useful in the treatment of comminuted fractures.

12

Conclusion

The hybrid locking compression plate constructs tested during this research show the

following trends:

• The remaining torque in each of the locking screw remained similar, even though

their placement changed

• Conventional screws loosened at a faster rate than the locking screws

• The dynamic stiffness of the construct was found as a function of number of

locking screws and where they were placed, proximal and distal end placement

was found to be an effective position

• Fixation efficacy was proposed to be a function of dynamic stiffness and may be

predicted using simple mathematic models, for the conditions investigated in this

study for a 2 cm osteotomy gap.

• Depletion of dynamic stiffness occurs with respect to time and cycles, therefore,

needs to be accounted for in surgery.

Acknowledgement

Rinki Goswami, Senior in Biological Engineering, Cornell University, edited the

manuscript.

13

References

1. Sheng H., Ching-C.H., J. L. Wang, Ching-Kong Chao, Jinn Lin., 2004. Mechanical

tests and finite element models for bone holding power of tibial locking screws,

Clinical Biomechanics 19, 738–745

2. Sudhakar K.V., 2005. Metallurgical investigation of a failure in 316L stainless steel

orthopaedic implant. Engineering Failure Analysis 12, 249-256

3. ASTM F136, 2002. Standard specification for wrought titanium-6 aluminum-

4 vanadium ELI (Extra Low Interstitial) alloy for surgical implant applications

(UNS R56401), ASTM International, West Conshohocken, PA

4. Miller D., Goswami T., 2007. A review of locking compression plate biomechanics

and their advantages as internal fixators in fracture healing. Clinical Biomechanics

22, 1049–1062.

5. Rozbruch, S.R., Muller, U., Gautier, E., Ganz, R., 1998. The evolution of

femoral shaft plating technique, Clin. Orthop., 195-208.

6. Gautier E, Sommer C., 2003.Guidelines for the clinical application of the LCP.

Injury 34, SB63–SB76

7. Stoffel K, Dieter U, Stachowiak G, Gachter A, Kuster M., 2003.Biomechanical

testing of the LCP – how can stability in locked internal fixators be controlled?

Injury 34, SB11–SB19.

8. Kubiak E, Fulkerson E, Strauss E, Egol K, 2006. The evolution of locked plates.

The Journal of Bone and Joint Surgery, 88-A, Supplement 4

9. Perren, S.M., 2001. Evolution and rational of locked internal fixator technology-

introductory remarks, Injury 32 (Suppl 2), S-B3-S-B9

14

10. Klaue, K., Fengels, I., Perren, S.M., 2000. Long-term effects of plate osteosyn-

thesis: comparison of four different plates, Injury 31, S-B51- S-B62

10. Patel, V., 2008. Biomechanical evaluation of locked and non-locked constructs

under axial and torsion loading, M.S. Thesis, Wright State University, Dayton, OH,

45435 USA.

11. Goswami, T, Patel, V. Dalstrom, D., and Prayson, MJ. 2011. Mechanical evaluation

of fourth-generation composite femur hybrid locking plate constructs, Materials in

Medicine, Journal of Materials Science, 22, 9, 2139-2146

12. Freeman AL, Tornetta P 3rd, Schmidt A, Bechtold J, Ricci W, Flemming M.2010,

How much do locked screws add to the fixation of hybrid” plate constructs in

osteoporotic bone? J Orthop Trauma. 24, (3), 163-169.

15

Table-1. Specifications of femurs in the experimental program

Group Femur No Screw-1 Screw-2 Screw-3 Screw-4

1 1,2,3,4,5 NL NL NL NL

2 1,2,3,4,5 NL NL NL L

3 1,2,3,4,5 L NL NL NL

4 1,2,3,4,5 L NL NL L

Notes, L: locking, NL: non-locking.

16

Table-2: Stress and displacement development in analytical models, ANSYS results

Femoral Construct Stress (N/m2) Displacement(m)

4 Non-Locking 449 6.3*10^-8

1 Locking 3 Non-Locking

427 7.9*10^-9

2 Non-Locking 2 Locking

373 5*10^-9

1 Non-Locking 3 locking

341 5.1*10^-11

4 Locking 190 3.8*10^-8

17

Table-3. Remaining torque (RT) measured in Nm, in each of the screws.

Femur type RT in Screw-1 RT in Screw-2 RT in Screw-3 RT in Screw-3

Gr.1, Femur-1 0 2.59 1.58 0.2

Gr.1, Femur-2 0 0 0 0

Gr. 1, Femur-3 0 0 0 0

Gr. 1, Femur-4 2.5 0.79 2.53 1.1

Gr. 1, Femur-5 0 0 0 0

Gr. 2, Femur-1 3.36 2.3 3.4 2.92

Gr. 2, Femur-2 2.5 3 0 2

Gr. 2, Femur-3 2.9 3 3.6 1.6

Gr. 2, Femur-4 2.58 3 3 3.7

Gr. 2, Femur-5 2.6 3 3 2.6

Gr. 3, Femur-1 3.6 2.6 2.6 3.38

Gr. 3, Femur-2 3.5 1.39 1.29 0.41

Gr. 3, Femur-3 0 0 0 0

Gr. 3, Femur-4 4 2.11 2 0

Gr. 3, Femur-5 2.6 0 0 0

Gr. 4, Femur-1 2.6 3.4 2.0 3.3

Gr. 4, Femur-2 2.57 2.39 2.05 2.56

Gr. 4, Femur-3 1.56 0 2.83 0.3

Gr. 4, Femur-4 3.31 2.3 3.4 1.4

Gr. 4, Femur-5 2.6 2 3.2 3.9

Color code- yellow- locked screws, green- non-locked screws

18

Table-4. Results of individual femur constructs showing average remaining torque (ART)

in Nm, percentage loosening (PL), average axial stiffness (AAS) in N/mm, average

torsional stiffness (ATS) in Nmm/ rotation, average deformation (AD) in mm and

observation on each test

Test No ART PL AAS ATS AD Observations

Gr.1, F-1 109 73 2400 51.8 0.35

Gr.1, F-2 0 100 1181 92.6 0.5 Failed at

44,3365 cycles

Gr. 1, F-3 0 100 398.5 334.8 0.54 Failed at 26,923

cycles

Gr. 1, F-4 1.73 56.75 603.6 175.6 0.74

Gr. 1, F-5 0 100 1022 118.8 0.78 Failed at 40721

cycles

Gr. 2, F-1 3 25.25 796.4 136.8 0.32

Gr. 2, F-2 1.88 53 555 168 1.12

Gr. 2, F-3 2.78 30.5 967 115.6 0.59

Gr. 2 F-4 3.07 23.25 812 147 0.82

Gr. 2, F-5 2.8 30 594 346 0.9

Gr. 3, F-1 3.04 23.9 630 165.6 1.46

Gr. 3, F-2 1.66 58.5 1213 78.7 0.3

Gr. 3, F-3 0 100 245 346 0.73 Screw-1 failed

Gr. 3, F-4 2.03 49.25 456 167 1.46

Gr. 3, F-5 0.65 84 556 239 0.97 Screw-1 pullout

19

Gr. 4, F-1 2.92 27 593 167 1.34

Gr. 4, F-2 2.39 40.25 337 331 1.006

Gr. 4, F-3 1.84 54 4011 27.67 0.05

Gr. 4, F-4 2.58 35.5 584 203 1.2

23

Figure 4: Finite element analysis results from ANSYS, one typical data sheet illustrated

above and numerical data tabulated in Table 2.