Forces, moments and temperatures are measured and transmitted by a multi-channel telemetry device. Results of the measurements with hip, knee, shoulder and spine implants are presented as video clips.

Severe compression fractures of a vertebral body or a tumour in the region of the spine sometimes require the replacement of a vertebral body by an implant. The loads on such an implant are not well known. In order to measure these loads, the commercially available vertebral body replacement ‘SYNEX’ was modified. It allows the in vivo measurement of three force components and three moments acting on the implant. The 9-channel telemetry transmitter developed in our biomechanics laboratory was placed into the cylinder of the implant together with 6 load sensors and a coil for the inductive power supply. Usually, the spine is in addition stabilized dorsally by an internal spinal fixation device implanted from the back side.

Coordinate system

The bone-based coordinate system was chosen according to ISO 2631. The x- axis in the median plane points anteriorly, the y-axis in the frontal plane to the left side, and the z-axis cranially.

The forces and moments are presented in the measuring units N and Nm.

Patients

WP1

WP2

WP3

WP4

WP5

Table with basic information about those patients who had vertebral body replacements:

Little was known about the loads acting on internal spinal fixators. In order to measure the loads, a commercially available implant was modified. A measuring cartridge was integrated into the longitudinal rod containing six load sensors, an 8-channel telemetry transmitter, and the secondary coil for the inductive power supply.

Both telemeterized fixators transmit their load values as a radio frequency pulse train outside the body. For the measurements, a flat power coil, fixed to the patient’s back, supplies the energy needed by both fixators. The power coil has an integrated antenna which delivers the signals to the external components of the telemetry system.

Coordinate system

The internal fixators were implanted pairwise. All reported data came from the left implant and are reported in a right-handed coordinate system.

The measured load components act at the centre of the cylindrical part of the implant. The z-axis is

the long axis of the fixator and points upwards. The y-axis is parallel to the axis of the Schanz screw and points ventrally. The x-axis is perpendicular to both others and is directed to the right side. All force components Fx, Fy, Fz act in axis directions while the moment components Mx, My, Mz turn clockwise around the axes.

Due to the anatomical conditions at the implantation site this coordinate system does not coincide exactly with the sagittal and frontal plane of the upper body. The forces and moments are presented in the measuring units N and Nm.

Patients

MS

NF

HS

FJ

JT

BB

JW

HB

LG

AG

Table with basic information about those patients who had instrumented spinal fixators:

In order to obtain realistic loading data, a knee implant with a 9-channel telemetry transmitter was developed which enables six-component load measurements in a primary total knee replacement. Both forces in axial, medio-lateral and anterio- posterior direction and flexion-extension, varus-valgus and internal-external moments can be measured.The instrumented knee joint is a modification of the INNEXTM System, Type FIXUC (Zimmer GmbH, Winterthur, Switzerland). The standard femur component and tibial insert are used. Only the tibial component was modified to enable the integration of the electronic devices. During modification of the tibial component, the patients’ safety was deemed to be especially important.

Coordinate system

The coordinate system of the instrumented knee implant is a

a right- handed coordinate system fixed at the right tibial implant (not at the bone!). If forces and moments are measured in a left knee, they are transformed to the right side. The coordinate system is located at the height of the lowest part of the polyethylene insert. The z-axis is aligned with the stem axis of the implant.

The force components +Fx, +Fy and +Fz act in lateral, anterior and superior direction on the tibial tray. The moment Mx acts in the sagittal plane of the tibial component and turns clockwise around the +x-axis. The moment My acts in the frontal plane and turns clockwise around the +y-axis and the moment Mz turns clockwise

around +z-axis in the transverse plane. A positive moment Mz acts if the tibial implant component (or the femur) rotates inwards and/or if the tibia bone rotates outwards. The OrthoLoad videos show the load componentsrelative to the tibial tray. The stem axis z of the tibial implant component is rotated backwards in the sagittal plane by about 7 degree relative to the long axis of the tibia bone. This slope of the implant varies inter-individually.

Patients

K1L

K2L

K3R

K4R

K5R

K6L

K7L

K8L

K9L

Table with basic information about the knee joint patients:

The picture shows an instrumented shoulder implant capable of measuring forces, moments and, in addition, the temperature acting in the glenohumeral joint. It was developed in the Biomechanics Lab of the Charité and contains a measuring unit with 6 semiconductor strain gauges and a 9-channel telemetry transmitter. Each strain gauge requires one channel of the telemetry while the remaining three channels are used for transmitting the temperature, the current supply voltage and a synchronising signal. At the lower end, an inductive coil ensures the power supply. The measuring signals are led with a pacemaker feed-through to the antenna (protected by a cap of PEEK) which transmits the signals to the external measuring unit.

Coordinate system

Humerus system

All loads are displayed as acting at the humerus. They are based on the ISB- recommended coordinate system (Wu et al., 2005) for the right shoulder joint. In this bone-based shoulder coordinate system, the positive x-Axis points in the anterior, the y-axis in the superior and the z-axis in the lateral direction. The moments Mx, My and Mz turn clockwise around the +x, +y and +z axes.

This system is right-handed for a right shoulder joint. For patient S3L, who obtained her implant on the left side, all values are mirrored to the right side to make it comparable to the other patients.

Implant system

In the implant-based coordinate system of the shoulder joint, the positive z-axis coincides with the neck of the implant and points in the medial- cranial direction. x- and y-axes are in the plane perpendicular to the implant neck. Axis x points laterally and y is oriented anteriorly. Load components relative to this implant-base system may be used to test fatigue or wear of implants, for example.

To obtain the forces and moments relative to the implant, the retroversion of the humeral head has to be known, indicated as α in the picture below. It can be measured relative to the anatomical landmarks of the epicondyles at the elbow or related to the orientation of the forearm in 90° elbow flexion as it is chosen during surgery (Hernigou et al., 2002). For some patients in OrthoLoad exact values for the retroversion to the epicondyles are available from a postoperative CT, taken for medical reasons. For the other patients a retroversion angle of 30° relative to the forearm in 90° elbow flexion was assumed as chosen by the surgeon during implantation.

The retroversion value for each patient can be found in the “Info Patient” window in OrthoLoad as the third rotation angle (picture below, right). In this example the given rotation angle of 63° corresponds to a retroversion angle of 27° (90°-63°). The other two angles are determined by the geometry of the implant and are therefore the same for all patients. The vector plot pictures (below, left) are simplified representations for better visualisation. The shown angle α is always the same and differs from the true angle in the patients.

General advice for the transformation of loads from a bone-based to an implant-based system is described here.

Scapula system

To obtain the loads relative to the scapula, a coordinate transformation would be required, taking into account the relative movement between humerus and scapula. This requires an accurate movement analysis. Such transformations are already planned but are not yet available.

Patients

S1R

S2R

S3L

S4R

S5R

S6R

S7R

S8R

Table with basic information about the shoulder joint patients:

Patient

Side

Gender

Weight [kg]

Height [cm]

Age at Implantation [years]

Indication

S1R

right

m

101

186

69

Osteoartheritis

S2R

right

m

85

161

61

Osteoartheritis

S3L

left

f

72

168

70

Osteoartheritis

S4R

right

f

50

154

80

Osteoartheritis

S5R

right

f

103

163

66

Osteoartheritis

S6R

right

m

135

186

50

Osteoartheritis

S7R

right

m

89

172

68

Osteoartheritis

S8R

right

m

83

173

72

Osteoartheritis

Literature:

Hernigou, P., Duparc, F., Hernigou, A., 2002. Determining humeral retroversion with computed tomography. J

Bone Joint Surg Am 84-A, 1753-1762 (http://www.ncbi.nlm.nih.gov/pubmed/12377904). Wu, G., van der Helm, F.C., Veeger, H.E., Makhsous, M., Van Roy, P., Anglin, C., Nagels, J., Karduna, A.R., McQuade, K., Wang, X., Werner, F.W., Buchholz, B., 2005. ISB recommendation on definitions of joint coordinate systems of various joints for the reporting of human joint motion–Part II: shoulder, elbow, wrist and hand. J Biomech 38, 981-992 (http://www.ncbi.nlm.nih.gov/pubmed/15844264).

The actual hip implant (Hip III) monitor the three force components and the three moment components acting on the ceramic head of the hip joint.

Instrumented implant

Hip III with one 9-channel transmitter

This new design of a instrumented hip implant was developed to measure contact forces and the friction at the joint in vivo. A clinical proven hip implant (‘Spotorno’ design) was modified in the neck area. The stem is build by TiAl6V4 and Al2O3- Ceramic was choosen for the implant head material. The neck was widened and enhanced with a 6.2 and 10mm hole. In the hollow neck are housed six semiconductor strain gauges, an internal induction coil and the telemetry. The six strain gauges are applied at the lower part on the inner wall (10mm hole) and connected to the 9-channel transmitter. The antenna, placed under the implant head, is connected by electronically feed-through to the internal telemetry. The feed-through is welded by a laser beam into the top plate. The hollowed neck is closed by the top plate and welded with an electron beam. Therefore the internal space is hermetically closed against the body fluids.

With this implant three contact forces acting onto the implant head center and three friction moments acting between the gliding partners can be measured in vivo.

Since April 2010 ten instrumented hip joints (Hip III) were implanted in ten patients (H1L, H2R, H3L, H4L, H5L, H6R, H7R, H8L, H9L and H10R) to monitor forces and moments. No further implantations are planned.

Coordinate system

Fermur system

All forces are reported in a right-handed coordinate system of the right femur (different from hip joint type I and II). The load components are reported as Fx, Fy, Fz. The femur system is fixed at the centre of the femoral head. The femoral midline (dotted black) intersects the axis of the neck in point P1. Point P2 is defined as the deepest point of the fossa intercondylaris at the distal end of the femur. The straight connection between P1 and P2 defines the z axis of the femur. The z axis of the coordinate system is parallel to the z axis of the femur.

The x axis of the coordinate system is defined perpendicular to z and parallel to a plane through the most dorsal parts of the condyles and points laterally. The y axis of the coordinate system is perpendicular to x and z and points ventrally.

Implant system

In order to test fatigue or strength of the implant itself, it may sometimes be required to know the force components in an implant-based coordinate system. Axis zi of this system coincides with the shaft axis of the implant. The xi axis lies in the neck-shaft-plane. For the transformation of forces from the femur- to the implant system, three angles are required: angle Sx between the z axis of the bone and the shaft axis of the implant, angle Sy between the z axis of the bone and the shaft axis of the implant and furthermore the anteversion angle AV of the implant. These data are provided by the table in the video (“Info Patient”).

Turning the system by +Sx around the – x axis

Turning the system by +Sy around the – y axis

Turning the system by -Av around the + z axis

More details about this transformation are given here and in Bergmann et al. (2001) (http://www.ncbi.nlm.nih.gov/pubmed/11410170?dopt=Abstract).

Patients

H1L

H2R

H3L

H4L

H5L

H6R

H7R

H8L

H9L

H10R

Table with basic information about the patients with Hip III implants:

Patient

Side

Gender

Weight [kg]

Height [cm]

Age at Implantation [years]

Indication

H1L

left

m

73

178

55

Coxarthrosis

H2R

right

m

75

172

61

Coxarthrosis

H3L

left

m

92

168

59

Coxarthrosis

H4L

left

m

85

178

50

Coxarthrosis

H5L

left

f

87

168

62

Coxarthrosis

H6R

right

m

84

176

68

Coxarthrosis

H7R

right

m

95

179

52

Coxarthrosis

H8L

left

m

80

178

55

Coxarthrosis

H9L

left

m

118

181

54

Coxarthrosis

H10R

right

f

98

162

53

Coxarthrosis

For the hip joint III, the forces and moments in an implant-based coordinate system are of especial interest. The torque around the shaft axis, for example, is one of the most important parameters for the stability of implant fixation. To transform the forces measured relative to the bone, as delivered by OrthoLoad, to the loads acting in the implant system, the anteversion angle AV of the implant, the CCD angle and the neck length L are required. This data is listed in the following table:

Ground reaction forces were measured using two 6 degrees of freedom force plates (AMTI, Watertown, MA). The coordinate system is a right-handed system.

Raw data:

VARIABLE

DESCRIPTION

UNITS

SAMPLE RATE

Fx,y,z 1/2

Ground reaction forces, forceplate 1 and 2

N

960Hz

Mx,y,z 1/2

Moments relative to the original forceplate coordinate system, forceplate 1 and 2

Nmm

960Hz

For the calculation of the center of pressure the following offsets were used. The true origins of the coordinate systems are located below the top surfaces, with a distance (zo).

The resultant bending moment Mbend , acts in the middle of the femoral neck and perpendicular to the neck axis, and is calculated with the following formula:

with

The forces (F_{x’}, F_{y’}, F_{z’}) and moments (M_{x’}, M_{y’}, M_{z’}) are measured in the “implant coordinate system” x’, y’, z’ centered in the middle of the implant head. The force component F_{x’} acts laterally, F_{y’} anteriorly, and –F_{z’} distally along the femur axis. The measured moment components M_{x’}, M_{y’}, and M_{z’} turn right around the x’, y’, and z’ axes. N is the distance between the head center and the middle of the femoral neck and is equal to ½ L.

M_{tne} acts around the neck axis of the femur and represents the torsional loading of the neck. It is calculated by α = -45° rotation of the “bone coordinate system” around the y-axis.

Two different hip implants (Hip I and Hip II) monitor the three force components acting on the ceramic head of the hip joint.

Instrumented implant

Hip I with one 4-channel transmitter

The implant is made of a titanium stem and a ceramic head. A compartment, 32 mm deep and 9.5 mm wide, houses the electronic instrumentation inside the neck of the prosthesis. Three semiconductor strain gauges were applied at the lower end of the inner wall and connected to the 4-channel transmitter. Two electrical feed-throughs, welded in the top plate by electron beam, form the transmitter antenna inside the ceramic ball. After the instrumentation, the top plate is welded by laser onto the prosthetic neck, thus sealing the inner space in such a manner that it is absolutely safe against the body.

Since 1988 four instrumented hip joints (Hip I) were implanted in three patients (EBL/EBR, JBR, IBL).

Hip II with two 8-channel transmitters

To get more information about a potential temperature increase of hip implant after longer walking distances, an implant with a hollow shaft was instrumented with two 8-channel telemetry transmitters. A common coil in the middle of the shaft supplies power to both telemetry circuits. Inside, eight temperature sensors are arranged along the whole neck and shaft. Three strain gauges placed inside the prosthetic neck monitor the three force components which act at the centre of the ceramic ball. A fourth strain gauge measures the strain of the stem.

One telemetry transmitter is placed inside the prosthetic neck; the second device is fixed inside the hollow shaft of the implant. A 4-lead feed through is welded by laser in the top plate of the neck and forms two single loop antennas for the signal transmission.

Since 1997 five instrumented hollow shaft hip joints (Hip II) were implanted in four patients (KWL/KWR, HSR, PFL, RHR).

Coordinate system

Femur system

All forces are reported in a right-handed coordinate system of the left femur (different from hip joint type: Hip III). The load components are reported as -Fx, -Fy, -Fz with negative signs. Positive values therefore indicate components acting toward the femoral head.

In many previously produced >OrthoLoad videos from the hip joint the minus signs are lacking!

The femur system is fixed at the centre of the femoral head. The femoral midline (dotted black/white) intersects with the axis of the neck in point P1. This midline leaves the femur distally at point P2. Point P2 is defined as the deepest point of the fossa intercondylaris at the distal end of the femur. The straight connection between P1 and P2 defines the z axis (marked in red). Perpendicular to z and parallel to a plane through the most dorsal parts of the condyles, the x axis is defined (green) and points medially. The y axis (blue) is perpendicular to x and z and points ventrally.

Implant system

In order to test fatigue or strength of the implant itself, it may sometimes be required to know the force components in an implant-based coordinate system. Axis zi of this system coincides with the shaft axis of the implant. The xi axis lies in the neck-shaft-plane. For the transformation of forces from the femur to the implant system, two angles are required: angle S between the z axis of the bone and the shaft axis of the implant, and the anteversion angle AV of the implant. This data is provided in a table.

Because the angle S is always small, transformation of the force components can be performed with sufficient accuracy by

Turning the system by +AV around +z axis

Turning the system by +S around the +x azis

More details about this transformation are given here and in Bergmann et al. (2001)(http://www.ncbi.nlm.nih.gov/pubmed/11410170?dopt=Abstract).

Patients

EBL/EBR (Hip I)

IBL (Hip I)

JBR (Hip I)

HSR (Hip II)

KWL/KWR (Hip II)

PFL (Hip II)

RHR (Hip II)

Table with basic information about the patients with Hip I and Hip II implants:

Patient

Implant

Side

Gender

Weight [kg]

Height [cm]

Age at Implantation [years]

Indication

EBL

Hip I

left

m

62

168

83

Osteoarthritis

EBR

Hip I

right

m

62

168

83

Osteoarthritis

IBL

Hip I

left

f

84

170

76

Osteoarthritis

JBR

Hip I

right

f

47

160

69

Femoral head necrosis

HSR

Hip II

right

m

82

174

55

Osteoarthritis

KWR

Hip II

right

m

72

165

61

Osteoarthritis

KWL

Hip II

left

m

72

165

61

Osteoarthritis

PFL

Hip II

left

m

98

175

49

Osteoarthritis

RHR

Hip II

right

f

60

N/A

63

Osteoarthritis

For the hip joint, the forces and moments in an implant-based coordinate system are of special interest. The torque around the shaft axis, for example, is one of the most important parameters for the stability of implant fixation. To transform the forces measured relative to the bone, as delivered by OrthoLoad, to the loads acting in the implant system, the anteversion angle AV of the implant, the CCD angle and the neck length L are required. This data are listed in the following table:

The loads (forces and moments) were measured in the implant-base coordinate system x’ y’ z’ (IBS). A force vector in the IBS is F’ = (Fx’, Fy’, Fz’), a moment vector is M’ = (Mx’, My’, Mz’). For the implant “HIP JOINT” the IBS was fixed to the left bone, for “HIP JOINT III”, “KNEE” and “SHOULDER” to the right bone. For subjects with implants at the opposite joint, the loads were first mirrored to the other side. Then they were mostly transformed to the bone-based system x y z (BBS). A force vector in the BBS is F = (Fx, Fy, Fz), a moment vector is M = (Mx, My, Mz). Only for the knee joint and the spinal implants they were left in the implant-based system x’ y’ z’.

The example shows the coordinate systems IBS and BBS of an implant of type “HIP JOINT III”. Definitions of the coordinate systems of the other implants are described in the manual under the caption “Implants”.

Relative to the BBS the implant is rotated three times in the order 1, 2, 3. The three rotation angles α1, α2, α3 are stated relative to the axes of the BBS! Their order and values are shown in the window “Info Patient” of the OrthoLoad videos:

In example 1 (implant type “HIP JOINT III”, right-sided implant) the three rotations and their order 1 2 3 are:

clockwise by angle α_{1} = α_{x }= +2° around axis +x

clockwise by angle α_{2} = α_{y }= -2° around axis +y

clockwise by angle α_{3} = α_{z }= -15° around axis +z

For this right-sided implant “HIP JOINT III”, a negative angle α_{z} indicates an anteversion of the implant neck.

In example 2 (implant type “HIP JOINT III”, right-sided implant) the three rotations and their order 1 2 3 are:

clockwise by angle α_{1} = α_{x }= +17° around axis +z

clockwise by angle α_{2} = α_{y }= +8° around axis +x

clockwise by angle α_{3} = α_{z }= 0° around axis +y

For this left-sided implant an anteversion is indicated by a positive angle α_{z}.

For transforming a force from the IBS x’ y’ z’ to the BBS x y z, three transformations have to be performed, using the transformation matrices T_{1} (α_{1}), T_{2}(α_{2}), T_{3}(α_{3}). The complete transformation matrix T’ is then:

T‘ = T_{3} (α_{3}) * T_{2}(α_{2}) * T_{1}(α_{1}) for F = T‘ * F’ (note the inverse order 3 2 1 of matrices!)

For transforming the loads from the BBS x y z to the IBS x’ y’ z’, this calculation has to be performed in the reverse order and with negative angles:

T = T_{1} (-α_{1}) * T_{2}(-α_{2}) * T_{3}(-α_{3}) for F’ = T * F

Rotations around the axes x y z are performed by these matrices:

For example 1 the complete transformation matrices T’, T are therefore:

T‘ = T_{z}(-15°) * T_{y} (-2°) * T_{x}(2°) for F = T‘ *F’

T = T_{x} (-2°) * T_{y}(+2°) * T_{z}(+15°) for F’ = T *F

For example 2 they are:

T’ = T_{y} (0°) * T_{x}(8°) * T_{z}(17°) for F = T‘ * F‘

T = T_{z} (-17°) * T_{x}(-8°) * T_{y}(0°) for F‘ = T * F

Transformations of the moments M and M’ are performed separately in an analogue way.

Measuring Units % Body Weight and Newton

In the OrthoLoad videos, the loads are mostly reported in %BW (percent of body weight) for the forces and %BW*m for the moments, except for the spine implants were they are stated in Newton. The subject’s body weight in Newton is stated in the window “Info Patient” (examples see above). To transform loads from %BW/ %BW*m to N / Nm, the forces / moments have to be multiplied by 1% of the body weight, in example 1 by 7.809 and in example 2 by 6.9.