Together with the knee the hip is the most heavily loaded joint. At higher age many people suffer from arthrosis which may require to replace the joint by an endoprosthesis. In Germany alone more than 200,000 artificial hip joints are implanted every year (Source: Statistisches Bundesamt, data from 2011). Different types of implant and methods for their fixation in the bone are used clinically. All of them have specific advantages and are selected depending on the needs of the individual patient.
In most cases patients can expect that their mobility becomes restored and the artificial hip joint will work well for at least 15 years. Occasionally, however, complication may occur after some years. They may be caused by very high forces acting in the joint or wear of the plastic component used to replace the joint socket. Extensive research is performed to further improve the implants themselves and their fixation in the bone. However, precondition for any further improvement of hip implants is to know the loads acting in this joint. The instrumented hip implants, developed by us, allow to measure the forces directly in the patient's joint. With the new hip implant (Hip III) also the three moments are measured.
Instrumented Hip Joint with 4-Channel Transmitter (Hip I)
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 gages were applied at the lower end of the inner wall and connected to the 4-channel transmitter. Two electrical feedthroughs, 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 absolutely safe against the body.
The implant monitors three force components and the temperature inside the neck [Graichen et al., 1991]. Since 1988 in vivo hip joint force measurements have been made with three patients, carrying four instrumented hip joints with our 4-channel transmitter (EBL/EBR, JBR, IBL).
Instrumented Hip Joint with two 8-Channel Transmitters (Hip II)
To get more information about a potential temperature increase of hip implant after longer walking distances, an implant with hollow shaft was instrumented with two 8-channel telemetry transmitters [Graichen et al., 1999].
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 gages placed inside the prosthetic neck monitors the three force components, which act at the center of the ceramic ball. A fourth strain gage 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 May 1997 five instrumented hollow shaft hip joints were implanted in four patients (KWL/KWR, HSR, PFL, RHR).
Instrumented Hip Joint with 9-Channel Transmitter (Hip III)
A new instrumented hip joint prosthesis was developed which allows the in vivo measurement of the 3 force and 3 moment components. A clinically proven standard implant was modified. Inside the hollow neck, six semiconductor strain gauges, a small coil for the inductive power supply and a 9-channel telemetry transmitter are applied. The neck cavity is closed by a titanium plate and hermetically sealed by electron beam welding. The sensor signals are pulse interval modulated (PIM) with a sampling rate of about 120 Hz. The pulses are transmitted at radio frequencies via a small antenna loop inside the ceramic head. The antenna wire is connected to the electronic circuit by a two-pin feed through [Damm et al., 2010].
Since April 2010 ten instrumented hip joints with 9-channel transmitter 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.
The measured forces in the hip joint are given in percent of the patient's body weight (%BW). The measuring accuracy of the instrumented implants is in the range of 1% to 2%.
- During walking the joint forces are typically around 250%BW (Video 754kB) [Bergmann et al., 1993] [Bergmann et al., 2001]. These loads vary individually, however. During slow jogging they increase to 500%BW or more.
- Standing on one leg loads the hip joint nearly as much as slow walking.
- When going up stairs, the forces in the joint are not much higher than during walking, but the torque acting on the implants is increased (Video 709kB) [Bergmann et al., 1995].
- When stumbling, extreme forces act in the joint, even without falling. Loads of more than eight times body weight were measured on such occasions (Video 631kB) [Bergmann et al., 2004].
- When riding a bicycle or using a home trainer, hip joint loading is typically only one third or half as high as during walking (Video 752kB). For cardiovasculat or general fitness training of patients with hip implants or arthrosis a home trainer is most appropriate.
- In elderly patients the use of crutches or canes unloads the hip joint less effectively than generally assumed.
- Soft shoes or soft floor material don't reduce the peak forces in the hip joint [Bergmann et al., 1995].
- When carrying a heavy weight in one hand during walking, the hip joint at the opposite side is loaded much more than at the side where the weight is hold [Bergmann et al., 1997].
Much more data from the loading of the hip joint are available from our database OrthoLoad.