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Os calcis fractures: a randomized trial comparing conservative treatment with impulse compression of the foot
Injury (1992)
23,
printed in Great Britain
(5), 30.5-307
305
OS calcis fractures: a randomized trial comparing conservative treatment with impulse compression of the foot M. W. H. Erdmann, J. Richardson and J. Templeton Department
of Orthopaedic
Surgery, North Staffordshire
Royal Infirmary, Stoke-on-Trent,
Published Aafa suggest that soft tissue involvemenf pk~ys a role in fhe aetiology of residual symptoms following fractures of the OScalcis This randomized study involved 23 patients with 24 ~intra-articukv fractures of the calcaneum. If was designed to evaluafe the efficacy of a pneumatic plantar impulse device in reducing pain, associated morbidity and convalescenf time. The results show a significant improvement in subtalar range of movement in the pumped group at 3 months, with pain significantly reduced at 6 months and at I year. There was no di&ence in weight bearing or walking distance between fhe pumped and control groups. The time from date of injuy to return to work WETsignifittzntlyreduced in the pumped group by an average of 3 months.
Introduction The OS &is is the most commonly fractured tarsal bone, accounting for 1-2 per cent of all diagnosed fractures (Burns, 1985). Calcaneal fractures are among the most disabling of all foot injuries, with a poor long-term prognosis directly related to involvement of the subtalar joint (Nade and Monahan, 1973). Intra-articular fractures often progress unsatisfactorily and opinion varies as to their most effective management. A proportion of patients continue to suffer pain and loss of function, whether the treatment is non-operative (Barnard and Odegard, 1970; Shannon and Murray, 1978; Pozo et al., 1984), by fracture reduction (Essex-Lopresti, 1952; Romash, 1988) or by primary arthrodesis (Zayer, 1969; Noble and McQuillan; 1979). Reports indicate that up to 20 per cent of patients are not back at work in 1 year (Pozo et al., 1984). Soft tissue involvement has been suggested as being significant in the aetiology of residual symptoms (Barnard and Odegard, 1970; Pozo et al., 1984). A trial was designed to investigate the hypothesis that cyclic impulse compression of soft tissues is of benefit in the treatment of OS calcis fractures. In the literature we have found few prospective trials (Barnard and Odegard, 1955; Romash, 1988) and no randomized, controlled trials comparing modes of treatment. This randomized study sets out to assess the effects of a cyclical compression foot pump, used in the first week after injury, on pain perception and return of function. 0 1992 Butterworth-Heinemann 0020-1383/92/050305-03
Ltd
UK
The A-V Impulse System The venous pumping mechanism in the foot, discovered by Gardner and Fox (1983), was shown to be activated by weight bearing and to operate independently of muscular action. The A-V Impulse System (Novamedix Ltd, UK) is an impulse pneumatic device which activates the plantar venous plexus by intermittent compression, thereby reproducing the action of weight bearing in individuals who are immobile. The apparatus comprises a self-contained air compressor, air reservoir and metering system, connected to an inflatable plastic pad which is placed in the plantar arch of the foot and intermittently inflated (Gardner et al., 1990). The device is unique in that it can be fitted within an immobilization cast. Studies with the mechanical foot pump applied to patients with leg trauma have been encouraging, showing rapid reduction in swelling and pain (MacEachem et al., 1985; Gardner et al., 1990). The pump has been shown to improve venous drainage and to enhance substantially arterial inflow into the leg (Morgan et al., 1990). It produces the same effect on venous pressure reduction as normal ambulation in the nonambulatory patient (McMullin et al., 1989).
Patients and method A series of 23 patients (T&kZ) with isolated intra-articular fractures of the OS &is seen in the casualty department were entered into the trial. They were randomized into a pumped or control group. All patients were given an explanation of the trial and required to sign an informed consent form. Extra-articular fractures of the calcaneum, multiple limb injuries and open fractures were excluded. All patients were admitted for a period of 1 week. The severity of each fracture was assessed to determine the indication for surgical reduction, based on radiological evidence of depression of the tuber-joint (Biihler’s) angle to less than 20” (Essex-Lopresti, 1952). Reduction was achieved in patients with depressed fractures within 24 h under general anaesthesia. A Gissane spike was inserted using image intensifier guidance and carried across the fracture, restoring the angle.
Injury: the British Journal of Accident Surgery (1992) Vol. 23/No.
306
Table 1. Basic clinical data for each treatment
group Control
Pumped Mean age (years) No. of patients Male Female Affected limb Left Right Bilateral
45.4 (19-66) 11 9 2
44.2 (20-75) 12 9 3 5 6 1
8 3 1
Table II. Summary of statistical results
Subtalar range of movement (“) Pain (VAS units) Return to work (months)
Time interval
Pumped
Control
3 months 6 months 1 year
57.5’ * 9.7 1 .1** 1 .l 0.8’ f 0.7
43.6 zt 15.6
4.3”
7.3
f 2.3
Statistically significant: ‘P-c 0.05, l‘P
Three patients (25 per cent) of the pumped group and four patients (36 per cent) of the control group required spike fixation. In those allocated to the pumped group, the inflatable pad was then secured in position and a plaster slipper moulded to the ankle allowing a subtalar range of movement. Patients with minimally depressed fractures (over 20”) were treated non-operatively with bed rest, with those in the pumped group having the pad applied to the foot over a protective tubinet stocking and a custom-made slipper fitted. All patients underwent a daily regimen of physiotherapy. The unit was left in operation continuously for I week. Suitable analgesia was provided, ranging from opiates in the first 24 h period to non-steroidal anti-inflammatory agents. Interestingly, although there was an initial period of adaptation lasting maximally 12 h, most patients experienced discomfort only when the pump was disconnected, and requested that it be reactivated. This observation correlates with findings in trials on tibia1 fractures using the pump (Gardner et al., 1990). At the end of the week all patients had a below-knee plaster cast applied and were discharged non-weight bearing. They were reviewed at I, 2, 3 and 6 months and at I year. At 1 month the cast was removed, as well as the Gissane spike if used, a lightweight patellar-bearing orthosis was applied and the patients encouraged to begin gradual weight bearing. At 2 months the orthosis was either removed or left for a further month, depending on their progress. The following parameters were measured. 1. S&alar movement. The range of eversion and inversion was gauged using an inclinometer, and the sum compared with that of the uninjured subtalar joint. The mean value of six measurements was then used for the trial. 2. Pain. A visual analogue score (VAS) graded from O-10 was used (Scott and Huskisson, 1976). 3. Wtighf beating. The ability to bear weight on the injured heel as a percentage of body weight was analysed. 4. Walking distance. The ability to walk a distance without
5
having to halt due to discomfort or pain was recorded using a treadmill. 5. Retttm to work. The time taken to return to their usual occupation or social. activity was noted. The patients were matched to controls. The degree of effort required for each occupation was assessed.
RemIts The two groups of .patients were matched for age and sex. One patient had sustained bilateral fractures of his calcanea, and had the pump applied to one foot with the other being the control. Stratified randomization was performed, with a block size of two, in two groups, control and pumped. Subtalar range of movement was analysed using the Student’s unpaired f test. All other results were analysed using the MannWhitney rank-sum test. There were significant improvements in the pumped group in subtalar range of movement at 3 months, pain perception at 6 months and at I year, and in return to work (Table II). The series of results obtained over the year are represented in Fipre 7 and Figwe 2. There was no significant difference in weight bearing or walking distance between the two groups. Three patients in the pumped group were able to bear partial weight at 1 month, and all were fully weight bearing by 6 months. All patients were able to walk at 3 months, and cover a minimum distance of I km without discomfort by 1 year. Complaints at 6 months were of persistent swelling, particularly over the lateral aspect of the heel and peroneal sheaths, and the inability to wear normal shoes. At 1 year, walking on level ground was generally not uncomfortable, but cobbled stones and steep inclines still proved difficult for some patients.
Discussion This trial has found a correlation between use of the foot pump in calcaneal fractures and improvement of pain and function. This is evidence of the importance of soft tissue management in these fractures. As calcaneal fractures are classically associated with
IOO-
75 -
I
0
I
I
I
I
,
1
I
12345678
I
9
I
10
I
11
,
12
Follow-uptmonths)
Figure I. Graphic representation range of movement (percentage footpump; 0, control.
of results obtained on subtalar relative to uninjured side). 0,
Erdmann et al.: OS calcis fractures
307
lOI-
References Anonymous (1987) EDRF. Lanret 2, 137. Anonymous (1988) Yin and Yang in vasomotor control. Lancet 2, 19. Barnard L. and Odegard J. K. (1955) Conservative approach in the treatment of fractures of the calcaneus. J Bone Joint Surg. 37A, 1231. Barnard L. and Odegard J. K. (1970) Conservative approach in the treatment of fractures of the calcaneus. 1. Bone Joint Surg. 52A, 1689.
0
12345678
9
10
11
12
Follow-upcmonths)
Figure2. Representation of pain as measured analogue score scale from 0 to 10. 0, footpump;
using a visual 0 control.
prolonged periods of immobilization in a plaster cast, we hypothesized that soft tissue swelling results in permanent stiffness around the sub&r joint. In our study, the pumped group displayed an improved range of subtalar movement
towards the end of the trial period, associated with reduced symptoms and improved function. The effect of the pump would not appear to be due to earlier weight bearing and better rehabilitation, as patients were not allowed to bear weight for 5 weeks following discharge. Timely use of the pump would appear to be important and we suggest that encouraging early weight bearing in addition could be of benefit in the management of these injuries. It is of interest that some of the more severe injuries seem to benefit particularly from use of the pump, in combination with Gissane spike reduction. The pathophysiological basis of the improved vascular dynamics is unclear, but the pump is believed to stimulate the release of endothelium-derived relaxing factor (EDRF), the endogenous vasodilator manufactured by the vascular endothelium. EDRF is a potent vasoactive peptide released by mechanical forces acting on the intima of blood vessels. It appears to be involved in flow-dependent dilatation, and to limit the pressure rise induced by an increase in blood flow (Anonymous, 1987,1988). In conclusion, a new method of treating calcaneal fractures has been presented, with a significant improvement in pain symptoms and return to work. The clinical signifi-
cance is reflected by the 3 month reduction in time off work. Although the number of patients in this study is small, there is no larger trial using this treatment. Application of the principle of soft tissue management results in shorter convalescence for patients with this most disabling of foot fractures.
Bums A. E. (1985) Fractures of the calcaneus - Symposium on osseous trauma of the foot. Clin. Podiafr. Med. Surg. 2, 311. Essex-Lopresti P. (1952) The mechanism, reduction technique, and results in fractures of the OS calcis. Br. J. Surg. 39, 3%. Gardner A. M. N. and Fox R. H. (1983) The venous pump of the human foot - preliminary report. Bristol Med. Chir. J 98, 109. Gardner A. M. N., Fox R. H., Lawrence C. et al. (1990) Reduction of post-traumatic swelling and compartment pressure by impulse compression of the foot. J Bone Joint Surg. 72B, 810. MacEachem A. G., Fox R. H., Gardner A. M. N. et al. (1985) The venous foot pump. British Orthopaedic Association autumn meeting. McMullin G., Scott H. J., Coleridge-Smith P. D. et al. (1989) An assessment of the effect of the foot pump on venous emptying in chronic venous insufficiency. In: Davy A. and Stemmer R. (eds). Phlebologie. London: John Libbey Eurotext Ltd, 69. Morgan R. H., Carolan G., Psaila J. V. et al. (1990) Arterial flow enhancement by impulse compression. I/kscttlar Surgery (in press). Nade S. and Monahan P. R. W. (1973) Fractures of the calcaneum: a study of the long-term prognosis. Injuy 4,201. Noble J. and McQuillan W. M. (1979) Early posterior subtalar fusion in the treatment of fractures of the OS calcis. 1. Bone joint Surg. 61B, 90. Pozo J. L., Kirwan E. O’G. and Jackson A. M. (1984) The long term results of conservative management of severely displaced fractures of the calcaneus. J. Bone Joint Surg. 66B, 386. Romash M. M. (1988) Calcaneal fractures: three-dimensional treatment. Foot Ankle 8, 180. Scott J. and Huskisson E. C. (1976) Graphic representation of pain. Pain 2, 175. Shannon F. T. and Murray A. M. (1978) OS calcis fractures treated by non-weight bearing exercises: a review of 65 patients. J. R. Coil. Surg. Edinb. 23, 355. Zayer M. (1969) Fracture of the calcaneus: a review of 110 fractures. Acta. Orthop. Stand. 40,530.
Paper accepted 10 October
1991.
Requests for reprints shoula’ be addressed to: Mr M. W. Erdmann, ‘Alderley’, 21 Lambridge Wood Road, Henley-on-Thames, Oxon RG9 3BP, UK.
23,
printed in Great Britain
(5), 30.5-307
305
OS calcis fractures: a randomized trial comparing conservative treatment with impulse compression of the foot M. W. H. Erdmann, J. Richardson and J. Templeton Department
of Orthopaedic
Surgery, North Staffordshire
Royal Infirmary, Stoke-on-Trent,
Published Aafa suggest that soft tissue involvemenf pk~ys a role in fhe aetiology of residual symptoms following fractures of the OScalcis This randomized study involved 23 patients with 24 ~intra-articukv fractures of the calcaneum. If was designed to evaluafe the efficacy of a pneumatic plantar impulse device in reducing pain, associated morbidity and convalescenf time. The results show a significant improvement in subtalar range of movement in the pumped group at 3 months, with pain significantly reduced at 6 months and at I year. There was no di&ence in weight bearing or walking distance between fhe pumped and control groups. The time from date of injuy to return to work WETsignifittzntlyreduced in the pumped group by an average of 3 months.
Introduction The OS &is is the most commonly fractured tarsal bone, accounting for 1-2 per cent of all diagnosed fractures (Burns, 1985). Calcaneal fractures are among the most disabling of all foot injuries, with a poor long-term prognosis directly related to involvement of the subtalar joint (Nade and Monahan, 1973). Intra-articular fractures often progress unsatisfactorily and opinion varies as to their most effective management. A proportion of patients continue to suffer pain and loss of function, whether the treatment is non-operative (Barnard and Odegard, 1970; Shannon and Murray, 1978; Pozo et al., 1984), by fracture reduction (Essex-Lopresti, 1952; Romash, 1988) or by primary arthrodesis (Zayer, 1969; Noble and McQuillan; 1979). Reports indicate that up to 20 per cent of patients are not back at work in 1 year (Pozo et al., 1984). Soft tissue involvement has been suggested as being significant in the aetiology of residual symptoms (Barnard and Odegard, 1970; Pozo et al., 1984). A trial was designed to investigate the hypothesis that cyclic impulse compression of soft tissues is of benefit in the treatment of OS calcis fractures. In the literature we have found few prospective trials (Barnard and Odegard, 1955; Romash, 1988) and no randomized, controlled trials comparing modes of treatment. This randomized study sets out to assess the effects of a cyclical compression foot pump, used in the first week after injury, on pain perception and return of function. 0 1992 Butterworth-Heinemann 0020-1383/92/050305-03
Ltd
UK
The A-V Impulse System The venous pumping mechanism in the foot, discovered by Gardner and Fox (1983), was shown to be activated by weight bearing and to operate independently of muscular action. The A-V Impulse System (Novamedix Ltd, UK) is an impulse pneumatic device which activates the plantar venous plexus by intermittent compression, thereby reproducing the action of weight bearing in individuals who are immobile. The apparatus comprises a self-contained air compressor, air reservoir and metering system, connected to an inflatable plastic pad which is placed in the plantar arch of the foot and intermittently inflated (Gardner et al., 1990). The device is unique in that it can be fitted within an immobilization cast. Studies with the mechanical foot pump applied to patients with leg trauma have been encouraging, showing rapid reduction in swelling and pain (MacEachem et al., 1985; Gardner et al., 1990). The pump has been shown to improve venous drainage and to enhance substantially arterial inflow into the leg (Morgan et al., 1990). It produces the same effect on venous pressure reduction as normal ambulation in the nonambulatory patient (McMullin et al., 1989).
Patients and method A series of 23 patients (T&kZ) with isolated intra-articular fractures of the OS &is seen in the casualty department were entered into the trial. They were randomized into a pumped or control group. All patients were given an explanation of the trial and required to sign an informed consent form. Extra-articular fractures of the calcaneum, multiple limb injuries and open fractures were excluded. All patients were admitted for a period of 1 week. The severity of each fracture was assessed to determine the indication for surgical reduction, based on radiological evidence of depression of the tuber-joint (Biihler’s) angle to less than 20” (Essex-Lopresti, 1952). Reduction was achieved in patients with depressed fractures within 24 h under general anaesthesia. A Gissane spike was inserted using image intensifier guidance and carried across the fracture, restoring the angle.
Injury: the British Journal of Accident Surgery (1992) Vol. 23/No.
306
Table 1. Basic clinical data for each treatment
group Control
Pumped Mean age (years) No. of patients Male Female Affected limb Left Right Bilateral
45.4 (19-66) 11 9 2
44.2 (20-75) 12 9 3 5 6 1
8 3 1
Table II. Summary of statistical results
Subtalar range of movement (“) Pain (VAS units) Return to work (months)
Time interval
Pumped
Control
3 months 6 months 1 year
57.5’ * 9.7 1 .1** 1 .l 0.8’ f 0.7
43.6 zt 15.6
4.3”
7.3
f 2.3
Statistically significant: ‘P-c 0.05, l‘P
Three patients (25 per cent) of the pumped group and four patients (36 per cent) of the control group required spike fixation. In those allocated to the pumped group, the inflatable pad was then secured in position and a plaster slipper moulded to the ankle allowing a subtalar range of movement. Patients with minimally depressed fractures (over 20”) were treated non-operatively with bed rest, with those in the pumped group having the pad applied to the foot over a protective tubinet stocking and a custom-made slipper fitted. All patients underwent a daily regimen of physiotherapy. The unit was left in operation continuously for I week. Suitable analgesia was provided, ranging from opiates in the first 24 h period to non-steroidal anti-inflammatory agents. Interestingly, although there was an initial period of adaptation lasting maximally 12 h, most patients experienced discomfort only when the pump was disconnected, and requested that it be reactivated. This observation correlates with findings in trials on tibia1 fractures using the pump (Gardner et al., 1990). At the end of the week all patients had a below-knee plaster cast applied and were discharged non-weight bearing. They were reviewed at I, 2, 3 and 6 months and at I year. At 1 month the cast was removed, as well as the Gissane spike if used, a lightweight patellar-bearing orthosis was applied and the patients encouraged to begin gradual weight bearing. At 2 months the orthosis was either removed or left for a further month, depending on their progress. The following parameters were measured. 1. S&alar movement. The range of eversion and inversion was gauged using an inclinometer, and the sum compared with that of the uninjured subtalar joint. The mean value of six measurements was then used for the trial. 2. Pain. A visual analogue score (VAS) graded from O-10 was used (Scott and Huskisson, 1976). 3. Wtighf beating. The ability to bear weight on the injured heel as a percentage of body weight was analysed. 4. Walking distance. The ability to walk a distance without
5
having to halt due to discomfort or pain was recorded using a treadmill. 5. Retttm to work. The time taken to return to their usual occupation or social. activity was noted. The patients were matched to controls. The degree of effort required for each occupation was assessed.
RemIts The two groups of .patients were matched for age and sex. One patient had sustained bilateral fractures of his calcanea, and had the pump applied to one foot with the other being the control. Stratified randomization was performed, with a block size of two, in two groups, control and pumped. Subtalar range of movement was analysed using the Student’s unpaired f test. All other results were analysed using the MannWhitney rank-sum test. There were significant improvements in the pumped group in subtalar range of movement at 3 months, pain perception at 6 months and at I year, and in return to work (Table II). The series of results obtained over the year are represented in Fipre 7 and Figwe 2. There was no significant difference in weight bearing or walking distance between the two groups. Three patients in the pumped group were able to bear partial weight at 1 month, and all were fully weight bearing by 6 months. All patients were able to walk at 3 months, and cover a minimum distance of I km without discomfort by 1 year. Complaints at 6 months were of persistent swelling, particularly over the lateral aspect of the heel and peroneal sheaths, and the inability to wear normal shoes. At 1 year, walking on level ground was generally not uncomfortable, but cobbled stones and steep inclines still proved difficult for some patients.
Discussion This trial has found a correlation between use of the foot pump in calcaneal fractures and improvement of pain and function. This is evidence of the importance of soft tissue management in these fractures. As calcaneal fractures are classically associated with
IOO-
75 -
I
0
I
I
I
I
,
1
I
12345678
I
9
I
10
I
11
,
12
Follow-uptmonths)
Figure I. Graphic representation range of movement (percentage footpump; 0, control.
of results obtained on subtalar relative to uninjured side). 0,
Erdmann et al.: OS calcis fractures
307
lOI-
References Anonymous (1987) EDRF. Lanret 2, 137. Anonymous (1988) Yin and Yang in vasomotor control. Lancet 2, 19. Barnard L. and Odegard J. K. (1955) Conservative approach in the treatment of fractures of the calcaneus. J Bone Joint Surg. 37A, 1231. Barnard L. and Odegard J. K. (1970) Conservative approach in the treatment of fractures of the calcaneus. 1. Bone Joint Surg. 52A, 1689.
0
12345678
9
10
11
12
Follow-upcmonths)
Figure2. Representation of pain as measured analogue score scale from 0 to 10. 0, footpump;
using a visual 0 control.
prolonged periods of immobilization in a plaster cast, we hypothesized that soft tissue swelling results in permanent stiffness around the sub&r joint. In our study, the pumped group displayed an improved range of subtalar movement
towards the end of the trial period, associated with reduced symptoms and improved function. The effect of the pump would not appear to be due to earlier weight bearing and better rehabilitation, as patients were not allowed to bear weight for 5 weeks following discharge. Timely use of the pump would appear to be important and we suggest that encouraging early weight bearing in addition could be of benefit in the management of these injuries. It is of interest that some of the more severe injuries seem to benefit particularly from use of the pump, in combination with Gissane spike reduction. The pathophysiological basis of the improved vascular dynamics is unclear, but the pump is believed to stimulate the release of endothelium-derived relaxing factor (EDRF), the endogenous vasodilator manufactured by the vascular endothelium. EDRF is a potent vasoactive peptide released by mechanical forces acting on the intima of blood vessels. It appears to be involved in flow-dependent dilatation, and to limit the pressure rise induced by an increase in blood flow (Anonymous, 1987,1988). In conclusion, a new method of treating calcaneal fractures has been presented, with a significant improvement in pain symptoms and return to work. The clinical signifi-
cance is reflected by the 3 month reduction in time off work. Although the number of patients in this study is small, there is no larger trial using this treatment. Application of the principle of soft tissue management results in shorter convalescence for patients with this most disabling of foot fractures.
Bums A. E. (1985) Fractures of the calcaneus - Symposium on osseous trauma of the foot. Clin. Podiafr. Med. Surg. 2, 311. Essex-Lopresti P. (1952) The mechanism, reduction technique, and results in fractures of the OS calcis. Br. J. Surg. 39, 3%. Gardner A. M. N. and Fox R. H. (1983) The venous pump of the human foot - preliminary report. Bristol Med. Chir. J 98, 109. Gardner A. M. N., Fox R. H., Lawrence C. et al. (1990) Reduction of post-traumatic swelling and compartment pressure by impulse compression of the foot. J Bone Joint Surg. 72B, 810. MacEachem A. G., Fox R. H., Gardner A. M. N. et al. (1985) The venous foot pump. British Orthopaedic Association autumn meeting. McMullin G., Scott H. J., Coleridge-Smith P. D. et al. (1989) An assessment of the effect of the foot pump on venous emptying in chronic venous insufficiency. In: Davy A. and Stemmer R. (eds). Phlebologie. London: John Libbey Eurotext Ltd, 69. Morgan R. H., Carolan G., Psaila J. V. et al. (1990) Arterial flow enhancement by impulse compression. I/kscttlar Surgery (in press). Nade S. and Monahan P. R. W. (1973) Fractures of the calcaneum: a study of the long-term prognosis. Injuy 4,201. Noble J. and McQuillan W. M. (1979) Early posterior subtalar fusion in the treatment of fractures of the OS calcis. 1. Bone joint Surg. 61B, 90. Pozo J. L., Kirwan E. O’G. and Jackson A. M. (1984) The long term results of conservative management of severely displaced fractures of the calcaneus. J. Bone Joint Surg. 66B, 386. Romash M. M. (1988) Calcaneal fractures: three-dimensional treatment. Foot Ankle 8, 180. Scott J. and Huskisson E. C. (1976) Graphic representation of pain. Pain 2, 175. Shannon F. T. and Murray A. M. (1978) OS calcis fractures treated by non-weight bearing exercises: a review of 65 patients. J. R. Coil. Surg. Edinb. 23, 355. Zayer M. (1969) Fracture of the calcaneus: a review of 110 fractures. Acta. Orthop. Stand. 40,530.
Paper accepted 10 October
1991.
Requests for reprints shoula’ be addressed to: Mr M. W. Erdmann, ‘Alderley’, 21 Lambridge Wood Road, Henley-on-Thames, Oxon RG9 3BP, UK.