Our purpose was to develop a fully instrumented tibial component
of a total knee prosthesis (Fig. 1) containing multichannel transducers
(load cells), a microtransmitter, and an antenna. Three complex
technologies were incorporated into a prosthesis capable of measuring in
vivo tibiofemoral compressive forces to transmit "real-time" data. The
geometry was modified to accommodate the transducer and electronic
technologies (Fig. 2).
As a result of the development of a fully instrumented implant,
direct load measurements were recorded intraoperatively during the
course of a total knee replacement. Long-term collection of in
vivo measurements of loads on a total knee prosthesis will
allow future improvements in knee-replacement design, rehabilitation,
and assistive devices such as lower-extremity braces, orthotics,
and shoes. This report describes the history and preliminary results
of the use of a total knee prosthesis with a telemetric implant,
the e-Knee. Until now no devices or methods for measuring loads in
vivo in the knee have been available.
Measurement of tibiofemoral forces is directly relevant to the design
and technique of implantation of knee prostheses. The wear and deformation
of the articulating surface (polyethylene), the distribution of
stress to the implant, the mechanical behavior of the bone-implant
interface, and the load-carrying characteristics of the remaining
bone are intimately related to the joint load. Until now, investigators
have relied principally on theoretical data from mathematical modeling,
biomechanical testing of cadavers, and clinical gait analysis to
predict the physical forces on the knee (Fig. 3).
Predictions based on theoretical data have revealed that tibiofemoral
forces range from three to six times body weight during walking,
ascending and descending stairs, and isokinetic exercise. While
the information gleaned from extensive historical studies has been
useful, each technique has substantial shortcomings. The information
generated by the e-Knee enables a direct comparison of measured
joint forces with those that are theoretically predicted. Intraoperative
measurements of joint loads may also be used to guide interrelated parameters
such as selection of prosthetic size, joint-line placement, and
ligament balancing. In addition to providing intraoperative measurements,
long-term implantation of an e-Knee will allow in-depth analysis
of forces transmitted across the knee joint during normal human
activity, such as walking, rising from a chair, and stair-climbing.
The results of this analysis can be used to develop design improvements,
refine surgical instrumentation, guide postoperative physical therapy,
and potentially detect the individual activities that would overload
the implant. Patients who are counseled regarding factors that overstress
the capacity of the prosthesis, such as running, jumping, lifting
heavy weights, and obesity, can benefit from modernization of data
to support the findings of previous reports. Telemetry has been
used to measure in vivo forces in the hip1,2, spine, and femur. The available
space within the knee prosthesis is physically more restricted than
that within the hip. A prototype to the e-Knee, a tibial prosthesis
instrumented with load cells, was demonstrated to be highly accurate
by Kaufman et al. in 19963. Testing
of this prosthesis demonstrated a correlation of 0.999 between actual
and calculated load. The same instrumented prosthesis was subsequently
used in a cadaver study (Fig. 4) to measure the effects of joint-line
elevation on tibiofemoral forces4.
On the basis of the biomechanical results, the instrumented model
was used to develop the configuration of the e-Knee.
This project represents an initiative undertaken by clinicians, scientists,
and industry beginning in 1993; each participant contributed his
or her established expertise. The geometry of an off-the-shelf prosthesis
was modified to contain multichannel load transducers, a microtransmitter,
and an antenna. The implant manufacturer (DePuy Johnson and Johnson,
Warsaw, Indiana) provided a titanium-alloy tibial plateau, the design
of which has been in clinical use since 1988. The plateau accepts standard,
commercially available tibial inserts. The stem portion, which has
the same outer proportions as the revision component, is hollow
to allow for the installation of the electronic hardware. A standard
polyethylene cap that was modified to accept the antenna is threaded
onto the distal end of the stem. The technology contained within
the implant is layered (stacked), and the implant is designed to
be as strong as traditional implants.
Load cells were designed and patented by a specialty engineering
firm (NK Biotechnical, Minneapolis, Minnesota) and are located at
the four quadrants (anteromedial, anterolateral, posteromedial,
and posterolateral) of the tibial tray (Fig. 5). The total force and the location
of the center of pressure can be determined by measuring the force
of each load cell.
The tibial/transducer tray is 7.5 mm thicker than that
of a standard prosthesis to accommodate the load cells. The size
of the tibial tray in the transverse plane corresponds exactly to
a standard 76 51-mm minimally constrained posterior-cruciate-retaining
prosthesis. The tibial plateau is composed of an upper and a lower
plate separated by four 2.5-mm square support posts with a height
of 0.5 mm. Each transducer can measure up to 200 lb (90.7 kg) per
load cell (Fig. 6.
6).
The microtransmitter, as shown in Figure 7, has been described by one of us
(C.P.T.)5. It is developed from
off-the-shelf surface mount analog/digital (ATD) integrated
circuits (StrainLink; Microstrain, Incorporated, Burlington, Vermont).
The microprocessor modulates the radiofrequency and controls the
signal multiplexer, the ATD converter, and the digital filter. It
is powered by an inductive coil, rather than a battery, coupled
from the device, and it transmits measurements into a computer in
a readable format. The external inductive coil is applied just distal
to the knee.
The antenna is similar to the power technology used in pacemakers.
Powering the e-Knee system is remote by magnetic near field coupling
to transfer power to the implanted coil within the prosthetic stem.
An external receiving coil is driven with AC current generating
an AC magnetic field (Fig. 8). The external coil is driven by
a power amplifier and a function generator.
The biomechanical laboratory at the Scripps Clinic has been utilizing
the prototype of the e-Knee to perform cadaveric studies evaluating
implant performance for the past ten years, and it developed the
test protocols from the historical studies. The clinical staff of
the Scripps Clinic worked in tandem with the three manufacturing
companies to design and pretest an implant that would clear all
clinical hurdles for implantation in patients.
The fully integrated implant has undergone a duration of ten million
cycles of fatigue testing with use of the modified ASTM (American
Society for Testing and Materials) test method. Following the performance
of safety testing and calibration, the implant was sterilized and
utilized to capture intraoperative joint loads from a single patient
to evaluate tibial forces. Real-time data were collected along the
following parameters: zero position (no active forces) with 10 and
15-mm tibial polyethylene inserts; 45°, 90°, and 100° of flexion; and
varus and valgus forces.
A routine revision total knee replacement was performed on the
left side of a seventy-nine-year-old, 167-lb (75.7-kg) man in January
2001. Verbal and written consent was obtained from the patient prior
to the intraoperative measurements. During the course of the surgical
procedure, the e-Knee was inserted into the tibial canal (Fig. 9) for the purpose
of measuring tibial forces. Transducer data were sampled at regular
intervals with use of standard tibial inserts of two different thicknesses
(10 and 15 mm) (Fig. 10). Following the collection of data,
the e-Knee was removed and was replaced with a standard revision
prosthesis.
This project brings electronic technology into the field of orthopaedic
implants. The most notable feature of the e-Knee is its ability
to measure in vivo tibiofemoral forces, a technological
breakthrough with promise of aiding in the modification of implant
design, intraoperative decisions, postoperative rehabilitation programs,
and assistive devices. Sufficient data were detected in the first intraoperative
attempt to measure telemetric forces. Testing will proceed as follows:
(1) comparison of the intraoperative data with those obtained in
previously proven cadaveric studies mirroring intraoperative procedures;
(2) comparison of in vivo forces with theoretical
joint forces; (3) permanent insertion into a total knee replacement
candidate; (4) measurement of lift-off versus excessive loading
of individual quadrants during activities of daily living; (5) comparison
of posterior-cruciate-retaining and sparing designs; and (6) potential evaluation
of mobile-bearing total knee prostheses.
Permanent in vivo generation can extend the
range of data to measurements of tibial forces, including shear
torque forces and those generated with asymmetric designs. The next,
permanently placed implant will alter long-term data, including
those obtained at intraoperative, early postoperative, and extended
postoperative evaluations. The first in vivo implant
produced accurate and reproducible data for measurement of tibiofemoral
forces.