Knee biomechanics during impact landing understanding injury mechanisms and developing prevention strategies 1

Results from the biomechanical impact testing of porcine and human cadaveric knee specimens using this platform confirmed anterior tibial translation and axial tibial rotation as key fac

KNEE BIOMECHANICS DURING IMPACT LANDING:

UNDERSTANDING INJURY MECHANISMS AND

DEVELOPING PREVENTION STRATEGIES

YEOW CHEN HUA

NATIONAL UNIVERSITY OF SINGAPORE

2009

KNEE BIOMECHANICS DURING IMPACT LANDING:

UNDERSTANDING INJURY MECHANISMS AND

DEVELOPING PREVENTION STRATEGIES

YEOW CHEN HUA

B.Eng.(Hons.), NUS

A THESIS SUBMITTED

FOR THE DEGREE OF DOCTOR OF PHILOSOPHY IN

BIOENGINEERING

TABLE OF CONTENTS

4.1 STAGE A - Understanding Biomechanics of Landing

4.1.1 Analysis of landing maneuvers performed by human

a Subject recruitment, anthropometric measurement

b Motion capture and force plate systems 31

d Processing of kinematics, kinetics and energetics data 33

4.2 STAGE B – Investigation of Knee Injury Mechanisms

4.2.1 Developing a test platform for application of simulated

a Specimen procurement and preparation 35

c Motion capture system and marker placement 39

e Data processing 40

4.2.3 Analysis of osteochondral damage in knee specimens 41

b Preparatory steps for general histology 42

c Histological staining methods 42

e Cartilage thickness measurement 43

f Cartilage volume quantification 44

4.2.4 Finite element model of the tibiofemoral joint 45

b Model mesh and material properties 46

c Loading and boundary conditions 47

d Validation of finite element model 48

4.2.5 Simulated landing impact to osteochondral explants with

a Explant extraction and preparation 48

b Impact protocol (to assess degenerative changes) 48

c Impact protocol (to assess effects of non-sustained and

4.2.6 Analysis of osteochondral explant damage and

f Cartilage thickness measurement 54

4.3 STAGE C - Development of Injury Prevention Strategies

4.3.1 Restraints to prevent anterior cruciate ligament failure

4.3.2 Development of a knee protection device 60

a Knee brace with anterior-sloped hinge 60

4.3.3 Evaluation of the knee protection device under normal

c Measurement of contact joint force of knee brace 65

d Processing of kinematics and kinetics data 65

4.3.4 Evaluation of the knee protection device under large

a Analysis of cadaveric knee specimens in unbraced and

b Processing of compressive force and kinematics data 67

5.1.1 Regression relationships of knee kinematics and kinetics

a Time profiles of knee kinematics, kinetics and

d Knee flexion angular velocity 72

5.1.2 Effect of landing height and technique on sagittal knee joint

d Knee flexion angular velocity 77

5.1.3 Differences in energy-dissipating strategies between landing

techniques in both sagittal and frontal planes 80

a Joint angles at initial contact 80

b Compressive force, posterior femoral displacement and

5.2.3 Osteochondral damage in porcine specimens 93

c Mankin score comparison between specimens 96

5.2.4 Osteochondral damage in human cadaveric specimens 97

e Mankin score comparison between regions 104

5.2.5 Finite element analysis of a tibiofemoral joint during

a Validation with experimental results 105

b Prediction of compressive force, relative anterior tibial translation and axial tibial rotation 106

c Prediction of peak anterior cruciate ligament and

5.2.6 Damage and degenerative changes in osteochondral

d Overall cartilage thickness 124

5.3 STAGE C - Development of Injury Prevention Strategies

5.3.1 Comparisons between unrestrained and restrained impact

d Relative anterior tibial translation 131

i Cartilage surface morphology and volume 136

j Subchondral bone plate thicknesss 138

5.3.2 Comparisons between unbraced and braced knee

a Peak ground reaction force and contact force 139

5.3.3 Comparisons between unbraced and braced knee

conditions during large simulated landing impact 143

b Peak compressive force and contact force 143

6.2 Investigation of knee injury mechanisms 161

6.3 Development of injury prevention strategies 192

ACKNOWLEDGMENTS

This work was funded by the grant entitled, ‘R175-000-062-112: Construction and validation of a dynamic 3D Finite Element (FE) model of the tibio-femoral joint’ from the Ministry of Education-Academic Research Fund, and co-supported by the Defence Medical and Environmental Research Institute

The work would not have been possible without the careful guidance from my supervisors, Prof James Goh and Dr Peter Lee I wish to thank them for their selfless support and mentoring throughout the course of my doctoral studies I would like to express my grateful thanks to the staff from the Division of Bioengineering, namely Assoc/Prof Toh, Annie, Millie, Dorothy, Ernest, Matthew, Yen Ping and Jasmine, who have bestowed help upon me in one way or another Many thanks to the friendly team from Department of Orthopaedic Surgery - Grace, Hazlan, Dominic, Siew Leng, Irene, Soon Chiong, Jamaliah and Amit Also to the DMERI staff, Chee Hoong, Serene, Jianzhong, Kok Yong, Kaizhen, Douglas and Lee Tong, it was fun knowing you all, thanks! And the NUSTEP colleagues, Elaine, Hock Hee, Wendy, Evi, Wan Ping, Eriza, Julee, Haifeng, Hongbin, Eugene See and Eugene Wong, I appreciate all your ‘combat service’ support! And to Joe and Alvin from the Impact Lab for lending

me their high-speed video cameras, and Chris Au from Functional Imaging Centre (NUH) for conducting the MRI scans, you guys are crucial in my project, thanks!

Not to forget my 9 precious FYP students, Kian Siang, Puay Yong, Ngee

CHAPTER 1 SUMMARY

~ Don't fear failure so much that you refuse to try new things

The saddest summary of a life contains three descriptions:

could have, might have, and should have ~

Louis E Boone

1 SUMMARY

In recreational and sporting activities, the landing maneuver is rather common

It involves rapid and high impact loads and is frequently associated with high knee injury risk These injurious loads can potentially lead to two common types of knee injury, namely anterior cruciate ligament (ACL) ruptures and articular cartilage lesions, which may accelerate the risk of developing secondary osteoarthritis (OA)

As these injuries can affect one’s mobility and quality of life due to abnormal locomotion and aggravated knee pain, there is a strong need to develop preventive measures to protect the knee joint from these impact loads

Currently, it is unclear how the biomechanical parameters that govern a landing maneuver may lead to these knee injury mechanisms, and how these injuries may give rise to joint degeneration towards OA The main aim of this project was to bridge these research gaps through the use of a wide range of experimental methods, which include motion analysis, biomechanical testing, bio-imaging, histological techniques, biochemical assays and finite element analysis, and to utilize this knowledge to devise a knee protection device to minimize knee injury risk during landing The project comprises of three stages: (A) understanding biomechanics of landing, (B) investigation of knee injury mechanisms, and (C) development of injury prevention strategies

The findings indicated substantial regression relationships of landing kinetics

guideline to develop a test protocol that can deliver a simulated landing impact to cadaveric knee specimens

Results from the biomechanical impact testing of porcine and human cadaveric knee specimens using this platform confirmed anterior tibial translation and axial tibial rotation as key factors in the ACL injury mechanism; the impact loads that resulted in ACL failure were also found to induce significant tibio-femoral cartilage damage The finite element analysis predicted an increase in ACL and tibial cartilage stresses with higher peak compressive impact Furthermore, the menisci-covered osteochondral regions, compared to exposed regions, demonstrated greater susceptibility to damage and degeneration when subjected to high impact stresses

Considerable inhibition of anterior tibial translation and axial tibial rotation was found to protect the ACL, but aggravate the damage in certain tibiofemoral osteochondral regions A knee protection device, based on an anterior-sloped hinge design, was then developed and tested on both human subjects and cadaveric knee specimens It exhibited greater efficacy in the mitigation of injury risk factors during normal landing performed by subjects, relative to the large simulated landing impact applied to cadaeric knees

Altogether, this project provided enhanced insights into the risk factors implicated in the knee injury mechanisms leading to ACL failure and osteochondral damage, and revealed prospective approaches to designing knee protective devices for attenuating injury risk

LIST OF TABLES

5.1.1 Regression equations and coefficients

5.1.2 Summary of mean eccentric work and percentage contribution to

total energy dissipation of the lower extremity joints

5.2.1 Experimental data of porcine knee specimens during impact

compression

5.2.2 A) Summary of displace magnitudes for impact compression trials

B) Experimental data of human cadaveric knee specimens during anterior cruciate ligament failure upon simulated landing impact 5.3.1 Experimental data of porcine knee specimens from unrestrained

and restrained test groups

LIST OF FIGURES

Figure Description

3.1 Flowchart illustrating the outline and flow of the research

4.2.1 A) Experimental setup for impact compression of porcine knee

B) Experimental setup for impact compression of cadaveric knee

4.2.2 Calculation of posterior femoral displacement and axial tibial rotation

4.2.3 Locations where osteochondral explants were extracted

4.2.4 Measurement of cartilage thickness from magnetic resonance imaging scans

4.2.5 A) Segmentation and stacking from MRI slices

B) Finite element model of the tibiofemoral joint

4.2.6 Schematic of setup to apply simulated landing impact to explants

4.2.7 Displacement compression curves for simulating landing impact

4.2.8 Differential zones of the knee articular cartilage

4.3.1 A) Unrestrained mode of the impact setup

B) Setup with relative anterior tibial translation restraint

C) Setup with axial tibial rotation restraint

D) Setup with both restraints

E) Photograph of restrained mode of impact setup

4.3.2 Locations where tibial osteochondral explants were extracted

4.3.3 Schematics of brace prototype

4.3.4 Schematics of shoe with carbon-fiber sole

4.3.5 A) Brooke shoes

B) Ossur 3DX GII ligament brace

4.3.6 Unbraced and braced modes of the impact setup

5.1.1 Profiles of ground reaction force, knee flexion angle, angular velocity and

joint power during landing phase

5.1.2 Regression relationship of peak ground reaction force with landing height 5.1.3 Regression relationships of knee flexion angles with landing height

5.1.4 Regression relationship of knee flexion angular velocity with landing height 5.1.5 Regression relationship of knee joint power with landing height

5.1.6 Profiles of ground reaction forces, knee flexion angles, angular velocities,

joint powers between single-leg and double-leg landing tasks

5.1.7 Comparison of peak ground reaction forces between landing height and

between landing techniques

5.1.8 Knee flexion angles at initial contact, at peak ground reaction force and at

maximum knee flexion during landing phase of single-leg and double-leg landing

5.1.9 Comparison of knee flexion angular velocities between landing height and

between landing techniques

5.1.10 Comparison of knee joint powers between landing height and between

landing techniques

5.1.11 Comparison of knee eccentric work between landing height and between

landing techniques

5.1.12 Comparison of hip, knee and ankle joint angles at initial contact between

double-leg and single-leg landing in both sagittal and frontal planes

5.1.13 Comparison of peak hip, knee and ankle joint angles between double-leg

and single-leg landing in both sagittal and frontal planes

5.1.14 Comparison of peak hip, knee and ankle joint angular velocities between

double-leg and single-leg landing in both sagittal and frontal planes

5.1.15 Comparison of peak hip, knee and ankle joint moments between double-leg

and single-leg landing in both sagittal and frontal planes

5.1.16 Comparison of peak hip, knee and ankle joint powers between double-leg

and single-leg landing in both sagittal and frontal planes

5.2.1 A) Dissection photograph and magnetic resonance imaging scans, pre-test

5.2.3 A) Key observations of cadaveric knee responses during simulated landing

impact

B) Specimen responses of a cadaveric knee during pre-failure and anterior cruciate ligament failure

5.2.4 A) Pre-test and post-test magnetic resonance imaging scans, and dissection

photograph of a human cadaveric knee specimen

B) Comparison of compressive force drop during pre-failure and anterior cruciate ligament failure trials

C) Gradual increase in compressive force drop with incremental

displacement impact loading

5.2.5 Photomicrographs of histological sections from control and post-impact

porcine knee specimens using Hematoxylin & Eosin and Safranin-O/Fast Green stainings

5.2.6 Mankin score distributions for each porcine specimen after simulated

landing impact

5.2.7 Comparison of Mankin scores between specimens and between sites

5.2.8 Comparison of cartilage volumes pre-test and post-test in different

compartments

5.2.9 Comparison of cartilage thicknesses pre-test and post-test in different

compartments

5.2.10 Photomicrographs of histological sections from control and post-impact

porcine cadaveric specimens using Hematoxylin & Eosin and O/Fast Green stainings

Safranin-5.2.11 Mankin score distributions for control and impacted cadaveric specimens 5.2.12 Comparison of Mankin scores between regions

5.2.13 Validation of model predictions

5.2.14 Model prediction of compressive force, posterior femoral displacement and

axial tibial rotation angle at 8-mm and 10-mm actuator displacements

5.2.15 Peak anterior cruciate ligament and tibial cartilage stresses during simulated

landing impact

5.2.16 Compressive stress response of the menisci-covered and exposed explant

groups at 1-mm and 2-mm displacements

5.2.17 Normalized cell viability profiles of control and impacted osteochondral

explants at different time points

5.2.18 Normalized glycoaminoglycan and collagen content profiles of control and

impacted osteochondral explants at different time points

5.2.19 Histological photomicrographs of control and impacted osteochondral

explants at different time points, using Hematoxylin & Eosin and O/Fast Green stainings

Safranin-5.2.20 Type I and II collagen immunohistochemical photographs of control and

impacted osteochondral explants at different time points

5.2.21 Comparison of Mankin scores of control and impacted osteochondral

explants at different time points

5.2.22 Comparison of normalized cartilage volumes of control and impacted

osteochondral explants at different time points

5.2.23 Comparison of peak impact stresses experienced by explants between

sustained and non-sustained conditions, and between cartilage regions

5.2.24 Photomicrographs of histological sections from the non-impact control,

sustained and non-sustained compression conditions, based on Hematoxylin

& Eosin and Safranin-O/Fast Green stains

5.2.25 Comparison of Mankin scores between the non-impact control, sustained

and non-sustained compression conditions, and between cartilage regions

5.2.26 Comparison of overall cartilage thicknesses between the non-impact control,

sustained and non-sustained compression conditions, and between cartilage regions

5.2.27 Comparison of zonal cartilage thicknesses between the non-impact control,

sustained and non-sustained compression conditions, and between cartilage regions

5.3.1 Photographs of anterior cruciate ligament status after final compression trial

for unrestrained and restrained test groups

5.3.4 Comparison of posterior femoral displacement between unrestrained and

restrained test groups

5.3.5 Comparison of axial tibial rotation angle between unrestrained and

restrained test groups

5.3.6 Photomicrographs of histological sections for the various tibial

osteochondral regions, based on Hematoxylin & Eosin and Safranin-O/Fast Green stainings

5.3.7 Comparison of Mankin scores between unrestrained and restrained test

groups across the various tibial osteochondral regions

5.3.8 Comparison of cartilage thicknesses between unrestrained and restrained test

groups across the various tibial osteochondral regions

5.3.9 A) 3D geometries of tibial osteochondral explants obtained through

reconstruction of the microCT scans

B) Comparison of cartilage volumes between unrestrained and restrained test groups across the various tibial osteochondral regions

5.3.10 Comparison of subchondral bone plate thicknesses between unrestrained and

restrained test groups across the various tibial osteochondral regions

5.3.11 A) Comparison of normalized peak ground reaction force between unbraced

and braced conditions

B) Contact force detected at the brace joint using Tekscan pressure film C) Stress distribution maps on the tibial component during landing phase of single-leg landing trials performed by three different subjects

5.3.12 Comparison of hip, knee and ankle flexion angles at initial contact, at peak

GRF and at maximum angle between unbraced and braced conditions

5.3.13 Comparison of normalized anterior tibial translation between unbraced and

braced conditions

5.3.14 Comparison of normalized axial tibial rotation between unbraced and braced

conditions

5.3.15 Post-impact magnetic resonance imaging scans and dissection photographs

to confirm presence of anterior cruciate ligament failure

5.3.16 Comparison of normalized peak compressive force between unbraced and

braced conditions

5.3.17 Comparison of normalized posterior femoral displacement and axial tibial

rotation angle between unbraced and braced conditions

5.3.18 Photomicrographs of histological sections from the control, unbraced and

braced conditions, using Hematoxylin & Eosin and Safranin-O/Fast Green staining

5.3.19 Comparison of Mankin scores between test groups at the different tibial

osteochondral regions from the control, unbraced and braced conditions

5.3.20 Comparison of cartilage thicknesses between test groups at the different

tibial osteochondral regions from the control, unbraced and braced

conditions

LIST OF ABBREVIATIONS AND SYMBOLS

ACL Anterior Cruciate Ligament

ATR Axial Tibial Rotation

ATT Anterior Tibial Translation

CFS Shoe with Carbon Fiber Sole

DMEM Dulbecco's Modified Eagle's Medium

Dx Posterior Femoral Displacement

ERP External-rotated position

Fdrop Compressive Force Drop

Finite Element FE

Fz Axial Compressive Force

GRF Ground Reaction Force

H&E Hematoxylin & Eosin

ICR Impact Compression with Axial Tibial Rotation Restraint

MicroCT Micro-Computed Tomography

MRI Magnetic Resonance Imaging

MTS Material Testing System

Saf-O/FG Safranin-O/Fast Green

UHMWPE Ultra High Molecular Weight Poly Ethylene

θz Axial Tibial Rotation Angle

CHAPTER 2 BACKGROUND

~ Man's unique reward, however, is that while animals survive by adjusting themselves to their background,

man survives by adjusting his background to himself ~

Ayn Rand

2 BACKGROUND

The Landing Maneuver

Landing is an essential athletic task, commonly employed during intensive sports activities like basketball, gymnastics and volleyball (Dufek and Bates, 1991; Hrysomallis, 2007; Marshall et al., 2007) This task is also performed in high-risk military activities like dismounting from trucks, running obstacle courses and parachute landing (Amoroso et al., 1997; Johnson, 2003) During landing, the double-leg landing strategy is preferred as it provides stability and minimizes injury risk in various activities that involve different magnitudes of ground reaction forces (GRF) However, lower extremity injuries, such as bone bruises and ligament tears, may result when excessive GRF is present (Dufek and Bates, 1991; Johnson, 2003; Kirkendall and Garrett, 2000; Frobell et al., 2008)

Landing Height

With the aim of identifying the factors involved in the mechanisms of landing impact injuries, previous studies have performed motion analysis work on double-leg landing from various landing heights, comparing between genders, between soft and stiff landing styles, and between gymnasts and recreational athletes (DeVita and Skelly, 1992; McNitt-Gray, 1993; Seegmiller and McCaw, 2003; Kernozek et al., 2005; Pappas et al., 2007) For instance, a double-leg landing study by DeVita and Skelly (1992) found that female volleyball and basketball players exhibited a peak GRF of 2-3 bodyweights (BW) during stiff and soft landing tasks from a 0.59-m height; their results suggested that soft landing (high knee flexion) reduced the impact stress on body tissues compared with stiff landing (low knee flexion) McNitt-Gray (1993) further tested landing heights of 0.32m, 0.72m and 1.28m for gymnasts and these subjects achieved peak GRF ranging from 3.9 to 11BW

Similarly, Seegmiller & McCaw (2003) illustrated that both gymnasts and recreational athletes exhibited an increase in the peak GRF (2.2-5.7 BW) with the tested landing heights (0.3m, 0.6m and 0.9m); furthermore, the gymnasts exhibited greater peak GRF than the recreational athletes during landing The large GRF experienced by athletes during landing from a great height may contribute to high injury risk (Quatman et al., 2006) Moreover, the knee joint is largely responsible for the body's ability to absorb shock during ground contact (DeVita and Skelly, 1992; Hargrave et al., 2003) The occurrence of low knee flexion during landing may lead to

a weaker knee joint power (DeVita and Skelly, 1992), which reflects diminished shock absorption capacity, and is likely associated with knee articular cartilage lesions (Hargrave et al., 2003; Lafortune et al., 1996; Derrick et al., 2002; Coventry et al., 2006; Jeffrey and Aspden, 2006) Additionally, Zhang et al (2000) demonstrated that

the knee joint power increased with landing height, which suggested that the knee joint is a major contributor to impact energy dissipation during landing

Though the reported GRF, knee flexion angles and joint powers from these previous landing studies were largely varied perhaps due to the different landing styles adopted by subjects, the collective results have implied that these parameters are closely linked to the landing height at which the task is executed; the greater the landing height, the larger the GRF, knee flexion angles and joint powers However, there is a limitation to the landing height at which the landing task can be safely performed in a controlled laboratory setting Hence, the magnitudes of these parameters are unknown at large landing heights beyond this limitation and yet this information is important for understanding lower extremity injury mechanisms Hence, it would be beneficial to establish regression relationships of peak GRF, knee flexion angles, angular velocities and joint powers with landing height, in order to facilitate the prediction of these parameters at large landing heights

Landing Techniques

Single-leg and double-leg landing techniques are common athletic maneuvers that are typically executed from various landing heights during intensive sports activities, such as basketball, volleyball, soccer and gymnastics (Dufek and Bates, 1991; Hrysomallis, 2007; Marshall et al., 2007)

These techniques are also present in high-risk military-related activities, such

as obstacle courses and parachute-landing falls (Amoroso et al., 1997; Gwinn et al., 2000; Johnson, 2003) These types of landing techniques have often been implicated

in the description of lower extremity injury mechanisms, especially anterior cruciate ligament (ACL) injuries, due to the presence of large ground impact, sudden deceleration and changes in movement direction (Dufek and Bates, 1991; Johnson, 2003; Griffin et al., 2000; McNitt-Gray, 1993)

A number of studies have examined the ground reaction forces (GRF) and the knee joint kinematics for these landing techniques, comparing between genders, between injured and non-injured subjects, and between fatigued and non-fatigued states Seegmiller and McCaw (2003) illustrated that an increase in landing height elevated the peak GRF based on double-leg landing techniques performed by female gymnasts from 0.3-m, 0.6-m and 0.9-m heights; they suggested that the incidence of lower extremity injuries may be attributed to the presence of large GRF experienced

by gymnasts during landing Gender-wise, Lephart et al (2002) found that female subjects tended to land with less knee flexion compared to males from a 0.2-m height during single-leg landing, which may explain the sex disparity in ACL injury rates

Recently, Pappas et al (2007) made a comparison between single-leg and double-leg landing techniques from a 0.4-m height, and revealed that both male and female subjects performed single-leg landing with significant differences in knee joint

kinematics, specifically reduced knee flexion, as compared to double-leg landing Altogether, these studies have illustrated that the type of landing technique can individually mediate the GRF and knee joint kinematics experienced by the subject However, it is still not clearly known how the knee joint will respond in terms of kinematics to the combined effects of different landing techniques.and heights

Moreover, knee energetics, such as joint power and eccentric work done, are important measures of energy absorption during landing A stiff double-leg landing was shown to exhibit less joint power and eccentric work done compared to a soft landing, which suggested a diminished capacity of the knee joint to dissipate impact energy (DeVita and Skelly, 1992) In addition, Zhang et al (2000) reported that the knee joint power and eccentric work done can elevate with increased landing height during double-leg landing and therefore augment energy dissipation Schmitz et al (2007) further demonstrated that females displayed less energy dissipation at the lower extremities during single-leg landing, compared to males Though these studies examined the effects of gender, landing height and landing stiffness on knee joint energetics, there is still a lack of understanding on how the knee joint may respond in terms of energy dissipation between single-leg landing and double-leg landing, yet this issue is important in explaining why single-leg landing leads to a higher injury risk relative to double-leg landing (Olsen et al., 2004) Therefore, it is important to examine the knee joint kinematics and energetics sustained during landing phase, in

Energy-dissipating Strategies during Landing

Landing-related injuries are common in intensive sports, such as volleyball, basketball and gymnastics (Dufek and Bates, 1991; Ferretti et al., 1992; Harringe et al., 2007; McKay et al., 2001) These injuries can be incurred from typical landing maneuvers, such as single-leg and double-leg landing, wherein the landing impact is attenuated primarily in the lower extremity joints (Coventry et al., 2006) The improper execution of the landing maneuvers can affect the shock attenuation capability of the lower extremity joints in response to the landing impact, which consequently aggravate joint loading and leads to injuries, like articular cartilage damage, ligament ruptures, bone bruises and menisci tears (Chen et al., 1998; Frobell

et al., 2008; Meyer and Haut, 2005; Sanders et al., 2000; Yeow et al., 2008)

The hip, knee and ankle joints contribute to shock absorption via eccentric work done by the joint muscles (Mizrahi and Susak, 1982) Previous studies have reported that these lower extremity contributions to energy dissipation can be influenced by gender type, landing height and landing stiffness DeVita and Skelly (1992) demonstrated that the hip and knee muscles were major contributors to energy dissipation during soft-style landing from a 0.59-m height; for a stiff-style landing, the ankle muscles absorbed more energy than the hip and knee muscles Furthermore, Zhang et al (2000) further illustrated that the hip and knee extensors served as major players in shock absorption during double-leg landing from heights of 0.32-1.03m Decker et al (2003) further found that the knee was the primary shock absorber for both genders during double-leg landing from a 0.6-m height; in addition, the ankle plantarflexors and the hip extensors were the second largest contributor to energy absorption for the females and males respectively

Several studies have investigated the lower extremity kinetics and kinematics

in both the sagittal and frontal planes during landing Kernozek et al (2005) revealed that most differences in kinematic and kinetic variables between male and female recreational athletes during double-leg landing (from a 0.6-m height) were observed

in the frontal plane, such as knee abduction angle and moment, rather than in the sagittal plane Moreover, Pappas et al (2007) showed that recreational athletes adopted greater knee abduction angle, lower hip adduction angle, reduced knee flexion angle at initial contact and peak knee flexion angle during single-leg landing from 0.4-m height, compared to double-leg landings Though there are several studies that have investigated the lower extremity energetics in the sagittal plane during landing (DeVita and Skelly, 1992; Zhang et al., 2000; Decker et al., 2003), little is known about the landing energetics in the frontal plane

Currently, there is a lack of understanding on the differences in energy dissipation strategies adopted by the lower extremity joints between single-leg and double-leg landing maneuvers Yet, these differences may be important in determining how a certain energy dissipation strategy executed for a particular landing maneuver may affect the injury risk compared to the other maneuver Thus, investigating and identifying the differences in sagittal and frontal plane energetics between double-leg and single-leg landing techniques would be crucial

Knee Injuries

Knee injuries pose a serious obstacle to one’s daily activities Majewski et al (2006) documented ~40% of the sports-related injuries to be at the knee joint The knee injury risk was ten times higher in recreational and competitive sports than in commuting and lifestyle activities (Haapasalo et al., 2007) These activities usually involve high and rapid impact, which are sustained by soft tissue structures, especially the anterior cruciate ligament (ACL) and the articular cartilage (Eckstein et al., 2005; Cochrane et al., 2007) The consequence of these damages would be an increased risk

of joint degeneration and potentially early onset of osteoarthritis (OA) (Nelson eta l., 2006; Trumble and Verheyden, 2004)

Anterior Cruciate Ligament (ACL) Injury

Anterior cruciate ligament injury is a major problem worldwide (Frobell-et

-al.,-2007;-Nielsen-et-al,-1991) and occurs in about 1 out of 3000 individuals annually

in the United States alone (Huston-et-al,-2000) It prevails in intensive sports, like basketball and volleyball (Dufek-and-Bates,-1991), that usually involve repetitive high landing impact loads (Gwinn-et-al.,-2000); these large loads can affect anterior tibial loading (Pflum-et-al.,-2004) which is an important contributor to ACL failure mechanism (DeMorat-et-al.,-2004;-Meyer-et-al.,-2005)

The ACL serves as a primary restraint to anterior tibial translation and a secondary restraint to axial tibial rotation (West and Harner, 2005) Numerous studies have indicated that both anterior tibial translation and axial tibial rotation are potential risk factors in the mechanism of non-contact ACL injury (Aune et al., 1997; DeMorat

et al., 2004; Kanamori et al., 2002; Sakane et al., 1997; Yeow et al., 2008) In-situ ACL forces were previously found to increase when anterior tibial loads were elevated from 22-110N (Sakane et al., 1997) Also, Aune et al (1997) introduced anterior tibial translation, together with quadriceps forces of 5-N and 0.9-kN magnitudes, in cadaveric knees to achieve ACL failure while DeMorat et al (2004) further demonstrated that an aggressive 4.5-kN quadriceps contraction led to significant anterior tibial translation of 19.5 mm that led to ACL failure

Externally-applied axial tibial torque was also found to increase in-situ ACL

Post-traumatic Osteoarthritis (OA)

Osteoarthritis (OA) is a prevalent joint disease that affects diarthrodial joints, especially the knee joint (Birchfield, 2001) In the United States, OA was named the second most common form of disability with nearly 1 in 3 adults showing OA joint symptoms (Birchfield, 2001; Endres et al., 2007) The wearing of cartilage in OA causes joint pain and may become so severe in advanced stages that patients’ mobility and quality of life are affected (Lorenz and Richter, 2006)

Secondary OA can occur due to trauma present in sports injuries (Birchfield, 2001; Vignon et al., 2006) It is also a long-term post-injury disorder that is often associated with ACL failure; its reported prevalence after ACL injury is up to 83% (Fink-et-al.,-2001;-Von-Porat-et-al.,-2004) wherein a majority of young patients suffers from early-onset OA with knee pain and impaired locomotion (Lohmander-et-

al.,-2004;-Von-Porat-et-al.,-2004)

Several studies illustrate that during ACL deficiency, the knee kinematics become abnormal and render certain cartilage regions to excessive stress and consequent degeneration (Andriacchi et al., 2006; Shefelbine et al., 2006) Furthermore, Li et al (2006) reported in an in-vivo study that during ACL deficiency, the tibiofemoral contact points shifted towards regions where degeneration was observed in patients with chronic ACL injuries Though altered kinematics following ACL injury can contribute to OA progression, it may not be the sole factor responsible for the initiation of joint degeneration The osteochondral lesions inflicted

by an injurious landing impact at the time of ACL failure may also accelerate the risk

of OA development

Osteochondral Lesions and Degeneration

High-impact activities such as gymnastics, volleyball and basketball, usually contribute large loads at the knee joint that may lead to articular cartilage lesions, hence impact trauma is a potential risk factor for joint degeneration and early onset of

OA (Vignon et al., 2006; Dufek and Bates, 1991; Wilk et al., 2006) Widuchowski et

al (2007) further reinforced this notion with evidence indicating that focal osteochondral lesions formed a 67% majority of documented cartilage lesions, with at least 45% connected with sports Moreover, Wilder et al (2002) indicated that individuals who incurred a traumatic knee injury are at least nine times more likely to develop knee OA than those who did not have a history of knee injury

These studies illustrated the deleterious effect of trauma on the articular cartilage, which may consequently increases the risk of joint degeneration and early-onset OA (Nelson et al., 2006; Trumble and Verheyden, 2004) Huser and Davies (2006) illustrated that a single-impact load on equine cartilage explants, based on a 0.5-kg weight dropped from heights up to 100-mm, led to degenerative changes, such

as increase in explant weight, release of glycoaminoglycans (GAG) and chondrocytic death, which were initiated within 48-h post-impact and increased with recovery time

in culture

Lahm et al (2005) and Mrosek et al (2006) further demonstrated that a

2.1-kN impact load applied to a canine knee model was able to induce subchondral

Subchondral traumatic lesions like bone bruises may also occur due to severe compressive impact loads at the time of ACL injury (Frobell-et-al.,-2008), hence they pose a vital risk factor for early-onset OA (Uhl-et-al.,-2005); bruise locations following ACL injury are also suggested to reflect ACL failure mechanism (Speer-et-

al.,-1995) Moreover, Seegmiller and McCaw (2003) suggested that high-impact loading present in athletes during landing raises concerns for repetitive stress injuries; the knee joint is the commonest affected joint that suffers from a high risk of overuse sports injuries due to deleterious impact (Lau et al., 2008) Prolonged impaction can induce crushing of subchondral bone, especially under high loading stress rates (Silyn-Roberts-and-Broom,-1990) Despite previous clinical imaging and cadaveric impact studies, there is still a lack of understanding on how the application of compressive impact loads leading to ACL injury may affect the extent and distribution of osteochondral lesions

Resilence of Different Osteochondral Regions to Loading

Different articular cartilage regions have varying levels of resilience to loading Thambyah et al (2006) found that menisci-covered cartilage exhibited considerably larger modulus by ~70%, and was thinner than exposed cartilage by

~40%; subchondral bone quantity and calcified layer thickness were also significantly lesser

Yeow et al (2008) further underscored that exposed regions do not generally sustain more damage than neighboring menisci-covered regions during a simulated landing impact of the knee joint; this observation suggested that the exposed cartilage region is more primed for load-bearing than its menisci-covered counterpart However, it is not fully understood whether these regions also possess different levels

of susceptibility to degeneration, which may be more relevant for understanding traumatic OA Hence, it is important to evaluate the damage and degenerative changes

post-at the various osteochondral regions after simulpost-ated landing impact

Mechanical Testing of Knee Specimens

Several studies have conducted mechanical tests on knee joint specimens to assess the mechanical factors influencing ACL failure during landing Aune et al (1997) applied an 889-N quadriceps force to inflict ACL failure during anterior tibial translation in cadaveric knees; DeMorat et al (2004) showed that an aggressive quadriceps contraction of 4500 N resulted in considerable anterior tibial translation up

to 19.5 mm, which led to ACL rupture

In addition, Meyer and Haut (2005) demonstrated that ACL failure can also arise due to significant relative posterior femoral displacement from excessive tibiofemoral joint compression Despite these studies, the extent of cartilage damage sustained upon ACL failure during simulated impact landing is currently not well-investigated

Recent studies have indicated that joint contact loads, during high impact activities, can induce cartilage deformation and damage, which may be the precursor for joint degeneration Thambyah et al (2005) applied a 4-bodyweights (BW) static compressive load to cadaveric knees at deep flexion and obtained contact stresses of

~25 MPa, which was suggested to be reaching the damage threshold of cartilage Furthermore, Lahm et al (2005) reported that a 2.1-kN transarticular impact in a canine knee model resulted in a subchondral fracture without cartilage damage, and 6-month post-trauma revealed degenerative changes such as reduced expression of collagen type-II and aggrecan (Mrosek et al., 2006)

However, the loading conditions in these studies were not relevant in simulating impact landing, hence the extent of cartilage injury documented will not be completely indicative of the potential damage sustained by the articular cartilage during ACL failure in impact landing Therefore, there is a strong need to develop a

test protocol that is able to apply a simulated landing impact to a knee specimen, and allow us to investigate the sole effect of landing impact loads on the induction of ACL failure, and the extent and distribution of tibiofemoral cartilage damage The test protocol will hold potential for future examination of the effectiveness of various bracings in attenuating ACL and cartilage injuries

Knee Landing Injury Mechanisms in relation to ACL and Cartilage Stresses

Excessive landing impact can induce substantial compressive knee joint loading, which can lead to articular cartilage lesions; the compressive impact further translates into anterior tibial shear forces via the posterior tibial slope (Pflum et al., 2004), and therefore promotes anterior tibial translation and axial tibial rotation, which increases ACL injury risk (Meyer and Haut, 2005; Yeow et al., 2008) The consequence of these injuries may likely be an accelerated progression of post-traumatic osteoarthritis (OA) (Lohmander et al., 2007)

In order to understand these injury mechanisms, it is important to examine the magnitude and distribution of impact stress at these soft tissues during the course of landing As stresses in the ACL and articular cartilage are not easily measured in in-vivo or in-vitro conditions, finite element analysis provides a potential approach to estimate the required stresses The finite element method involves mesh discretization

of a continuous domain into a set of discrete sub-domains (known as elements), allows detailed visualization of where structures deform, and indicates the distribution

of stresses and displacements Hence, a validated finite element model of the tibiofemoral joint would be necessary for estimating peak ACL and tibial cartilage stresses during a simulated landing impact

Mechanical Testing of Osteochondral Explants

Previous studies have investigated the extent of damage in cartilage explants after impact (Jeffrey et al., 1995; Huser and Davies, 2006; Morel and Quinn, 2004; Quinn et al., 2001) Quinn et al (2001) reported that matrix disruption was most severe at the superficial zone after impact, which might contribute to the increased release of proteoglycan Moreover, Morel and Quinn (2004) found that a single impact stress of 14MPa inflicted cell death and matrix cracks at the superficial zone, with a gradual increase in cell death and loss of demarcation between cartilage zones over a period of 11 days post-impact In addition, Huser and Davies (2006) applied a single impact load to equine cartilage explants and observed considerable surface damage, together with proteoglycan loss and chondrocyte death Collectively, these studies have indicated that impact loading can significantly damage the superficial cartilage zone However, the loading conditions adopted were not simulating landing impact; hence their findings may be less reflective of the extent of cartilage damage sustained during injurious landing maneuvers

Furthermore, the structural changes of the articular cartilage at the point of peak displacement compression during a landing impact are still unknown Yet this information is more representative of the actual damage and deformation afflicted by the impact stress Hence, it is important to examine the differences in the extent of damage and deformation at the tibial cartilage between two events: 1) at peak