C. Roman `o2, M. Ferrarin1, M. Rabbuffetti1, M. Recalcati1, E. Meani2
1SAFLo Gait Laboratory – Don Gnocchi Foundation Research Center – Milan, Italy
2Department for Treatment of Osteoarticular Septic Complications (COS) – Director: Prof. Enzo Meani
Operative Unit for Septic Prosthesis – Responsible: Prof. Carlo L. Roman`o
“G. Pini” Orthopaedic Institute, Milan, Italy
Introduction
Two-stage revision is one of the most widely accepted procedures to eradicate infec- tion and restore function in infected hip prosthesis [4 – 6, 7, 12]. However no objective data have yet been published regarding the function of the operated limb and the abil- ity of the patients to walk during the period of permanence of the antibiotic-loaded spacer implant, that may range from few weeks to several months [8, 11]. The avail- ability of a preformed hip spacers allows reproducibility of the intervention and known mechanical resistance of the implant [1 – 3] making possible, in the majority of cases, protected weight bearing and walking while the spacer is in situ [10, 11].
In this preliminary report we summarize some of the findings that we obtained with computerized gait analysis in patients with hip spacers implanted for hip pros- thesis infection.
Materials and Methods
Gait analysis was performed in four patients (one male, 75 yrs, 180 cm, 85 kg; one female 53 yrs, 174 cm, 98 kg; one male, 25 yrs, 182 cm, 84 kg; one female, 65 yrs, 168 cm, 78 kg) in which an infected hip prosthesis had been previously removed and a preformed antibiotic-loaded spacer (Spacer-G – Tecres Spa, Italy) implanted.
Spacer-G is an off-the-shelf preformed antibiotic-loaded cement hip spacer. It is available in three different head sizes and two stem lengths, short (110 mm) and long (210 mm), that may be intra-operatively chosen. The inner part of the spacer features a stainless still rod that provides mechanical resistance. The cement is pre-loaded by the manufacturer with gentamicin at a concentration of 1.9 % (w/w).
All the hips in this series were operated through a lateral approach, with the patient laying in supine position. Two patients received a short-stem spacer, and two a long-stem spacer. In all the cases the spacer was fixed in the proximal part, to pre- vent implant rotation, with one pack of antibiotic-loaded cement (Cemex Genta – Tecres Spa, Italy) containing gentamicin 2.5 % and vancomycin 5 %. All patients were allowed touch-down weight bearing for one month and then partial weight bearing on the operated extremity with the assistance of two canadian crutches. Gait analysis was performed 6 to 10 weeks after spacer implantation.
dynamometric platform (Kisler, Winterthur, Switzerland) for ground reaction force (GRF) measurement and a telemetric 8-channel system (TELEMG, BTS, Italy) for EMG signals acquisition.
Kinematic data were recorded by means of 8 TV cameras located around the cali- brated volume of 2.5 × 1 × 2 m3, the three-dimensional position of subject’s relevant body segments was determined by means of reflective markers (10 mm in diameter) glued on the following bony landmarks: sacrum, seventh thoracic vertebra, seventh cervical vertebra and, on both sides of the body, posterior superior iliac spines, lateral femoral condyles, lateral malleoli and fifth metatarsal heads (Fig. 2). After data acqui- sition (sampling frequency: 50 frames/sec), the marker coordinates were low-pass fil- tered (cut-off frequency 3 – 7 Hz) and individual anthropometric parameters were used to estimate the internal joint centers, according to a kinematic skeletal model (Fig. 2). These, in turn, enabled computation of trunk and lower limbs kinematics.
Combining GRF data and kinematic data, lower limb joint moment and power were computed through inverse dynamic modelling. Surface EMGs were recorded using a telemetric 8-channel system (TELEMG, BTS, Milan, Italy) from Rectus Femoris (RF), Vastus Medialis (VM), SemiMembranosus (SM), Biceps Femoris Caput Longus (BFCL) e Tensor Fasciae Latae (TFL) muscles of the affected leg (implanted with hip spacer). Myoelectric signals were collected by pre-amplified Ag/AgCl electrodes (diameter: 25 mm, bipolar configuration, inter-electrode distance: 20 mm) and band- pass filtered (high-pass= 10 Hz, flow-pass= 200 Hz). Dynamic data and EMG signals were acquired at a sampling frequency of 1000 Hz and a resolution of 12 bit.
Fig. 1. Picture of the SAFLo lab. TV cameras and PC- based work station for data acquisition and processing are highlighted.
Fig. 2. Skeletal model of lower limb and pel- vis and configuration of markers acquired with the motion cap- ture system during the right limb stance phase of walking. The ground reaction forces (yellow arrow) mea- sured by the force platform is superim- posed.
The experimental protocol included: an upright quite standing posture, 10 walking trials where the platform was hit with one foot (5 trials for each foot) at natural walk- ing speed, 4 walking trials where the platform was hit with one crutch (2 for each crutch). With the above protocol, the analysis of the load distribution between upper/
lower limbs and between affected/unaffected side was allowed.
Specific data elaboration, performed with dedicated software developed by the Bioengineering Centre of the Don Gnocchi Foundation, provided gait parameters (mean gait velocity, cadence, stride and step length) and the time course of kinematic and dynamic variables (joint angles, moments and powers, absolute orientation of lower limb segments, pelvis and trunk.
Results
Kinematic Data
Kinematic data showed walking parameters that remained in the normal values, although in the slowest speed range. Mean velocity of the gait was in fact reduced from 20 % to 60 %, compared to normal subjects, in different patients. Reduction of mean velocity was due to an absolute reduction of the frequency and of the step and stride lengths. Compared to normal subjects, for the same velocity, we observed that the decrease of the cadence speed was mainly due to a decrease of the stride and step length, while the frequency was slightly increased.
Step lengths were asymmetrical, with an increase of the affected side and a decrease in the sound limb. Stance time and double support were also asymmetrical with an increase of the duration on the affected limb and a decrease on the contralat- eral side (Fig. 3).
Fig. 3. Patient M1. In blue the limb with the spacer; in red the sound limb. The graphics show: Normalized Stance Time – Normal- ized Swing Time – Nor- malized Double Sup- port Time. Stance phase is decreased in the affected side, while double support is pro- longed.
Gait strategy may differ:
) Two-phase walking:
– swing phase of the affected limb at the same time of the two crutches, – swing of the sound limb while the affected limb and the two crutches are sup-
porting body weight.
) Three-phase walking:
– Both two crutches are put forward
– The affected limb is put forward, between the two crutches that are hold still – The sound limb is then put forward, slightly after the two crutches and the
affected limb
Ground Reaction Forces
Ground reaction forces revealed that patients reduced vertical charge on the affected limb to 30 % – 50 % of body weight (Fig. 4), sharing the remaining weight equally to both crutches. The sound limb showed ground reaction forces in the normal range.
Kinetic Data and EMG
Postural adjustments and the use of crutches allowed the patients to reduce to nearly zero the frontal (Fig. 5) and sagittal moments around the affected hip, where only a very reduced extensor moments may be appreciated. Moments around the contralat- eral side are in the normal range. Calculated powers at the hip, knee and ankle joints in the affected limb are also near zero or zero. In this way stresses around the implant seems to be minimized and the requirements for abductor muscles activation are very low. According to this finding the electromyographic pattern of the recorded muscles of the thigh was normal as to concern timing and duration of activity, but recorded intensities were generally lower than in the contralateral side.
a b
Fig. 4. Patient M1. Ground reaction forces in the affected side (a) is reduced to approximately 1/3 compared to normal side (b).
a b
Fig. 5. Patient M1. Frontal moments around the affected hip (a) are reduced to nearly zero, while they remain in the normal range in the contralateral hip (b).
Joint Range of Motion
Joint range of motion was reduced at the hip, knee and ankle level from approxi- mately 10 % to 50 %, compared with normal subjects and with the contralateral side.
As to regard the timing, compared to normal subjects, the knee may anticipate flexion to start the swing phase. Hip extension is normal or slightly reduced and pelvis rota- tions are increased.
Discussion
This is the first report on gait analysis performed in patients that underwent hip spacer implant following the removal of a septic hip prosthesis.
Our data show that a well centered preformed hip spacer (no patients with spacer dislocation were included in the study), allows walking with two crutches with partial
operated limb, as it is observed in other conditions that affect the hip [9]. At the same time postural adjustments and weight balance between crutches and the sound limb reduce the ground reaction forces on the operated limb to 30 – 50 % of body weight, while moments and power around the hip spacers are decreased to nearly zero. In this way the patients are able to dramatically reduce the weight and muscular forces that act on the spacer implant, while the joint range of motion at the hip, knee and ankle is maintained, although reduced. This findings point out the ability of the body to provide useful postural modification, even in the absence of the proprioceptive input from a normal hip and even in the absence of a traditional total hip replacement. It is worth to note that the strategies put in place to reduce forces and moments around the operated hip (and ipsilateral knee and ankle) lead to a protection of the implant but they also may reduce muscular activation, leading, in the long run, to a progres- sive muscular atrophy. For this reason, even if a normal pattern of muscular activity was electromyographically recorded in our patients, the revision of the spacer should be performed as early as possible to prevent any unnecessary muscle wasting.
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