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Outcome Measures in Joint Replacement by Physical Activity Monitoring: Pre-pilot Study

Success rate figures for total joint arthroplasty are typically based on revision rates, and do not reflect patients’ satisfaction or quality of life. Neither do they take into account the patient’s physical mobility after a surgical procedure.1 In order to reduce the need for revision, many studies have focused on identifying reasons for the failure of total hip replacements (THR). Aseptic loosening is well recognized as having a significant impact on the longevity of THRs.2 Various factors such as age, gender, body mass index, activity level3,4,5 and thickness of the cement mantle6 may predispose people to aseptic loosening. Sochart7 recommended ongoing radiological assessment of the hip following THR, so that closer follow-up can be instigated as soon as loosening or osteolysis becomes apparent on x-ray.

There is evidence to suggest that physical activity (PA) helps to maintain cortical bone after THR,8 which is important as bone loss can be a contributor to aseptic loosening.9 However, activity levels can also be thought of as a measure of the frequency and intensity of use of the prosthesis, and there is evidence to suggest that a too-high level of physical activity is a risk factor for early prosthetic failure following THR, especially in younger patients4,10 and in particular, younger men.11 Assuming that PA reflects clinical outcome, we could think of assessing success or failure of the surgical intervention by simply measuring the mobility post-operatively, comparing against pre-operative performance and prescribing patient specific activity targets. The PA data will enable the primary physicians to triage the process of optimal referral to orthopaedic surgeons or physiotherapists. The optimal time for the joint replacement surgery can be defined by constant monitoring of trends in the patient’s level of activity, avoiding costly delays and premature surgery, and thus posing a risk of multiple revisions throughout the patient’s life.

The purpose of this study was 1) to evaluate the feasibility of acquisition of mobility data of individual patients undergoing THR; 2) to provide a basis for an informed decision of what self-reporting instruments should be proposed for the large pilot study of a newly developing device for documenting PA levels in patients following THR and Total Knee Replacement (TKR); 3) to detect possible trends in the PA data sets and advise what regression trend would describe the mobility data patterns and 4) to compare self-reporting instruments used in this study with recorded PA data to evaluate potential similarities and discrepancies in order to highlight potentials for an alternative method of functional outcome monitoring using PA data.

Materials and Methods

This was a pre-pilot study in preparation for a large prospective randomized clinical trial testing a proprietary PA monitor. The pre-pilot study was designed as a case series of four patients undergoing surgical treatment THR and monitored for their level of physical activity for four to 12 weeks. Patients were selected according to the inclusion criteria, which were, 1) patients referred to an orthopaedic surgeon for THR for osteoarthritis or rheumatoid arthritis of the hip; 2) age 60 to 79 years and 3) Body Mass Index between 19.5 and 29.9. The proposed sample size for this pre-pilot study was two men and two women.

Activity levels were recorded using the activPALTM professional one axis accelerometer. (Professional Activity Logger, PAL Technologies Ltd., Glasgow, U.K.) This is a commercially available motion sensor, dimensioned 35mm x 53mm x 7mmand weighing 20 grams. The device is attached to the wearer’s thigh using a PALstickiesTM double sided hydrogel adhesive pad. The device is equipped with a piezoresistive accelerometer sensor, capable of detecting acceleration in one dimension of up to 2G with a sampling frequency of 10Hz and recording interval of 0.1s.  The device records a person’s daily physical activity and can log activity for up to seven days at a time.  The data is retrieved and processed using a personal computer. A custom-written software applies a proprietary algorithm to the data so that the activity can be classified into periods spent sitting, standing and walking (stepping). The system assumes that periods of sedentary activity (sitting and lying) can be classified as inactive and all upright activities as active. According to the manufacturer’s listed output characteristic, the activPAL professional accelerometer provides activity duration analysis and stepping time by cadence with resolution of 1s and steps taken with an accuracy of 99 percent with the resolution of step interval one step/minute.


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