Aims

In total knee arthroplasty (TKA), blood loss continues internally after surgery is complete. Typically, the total loss over 48 postoperative hours can be around 1,300 ml, with most occurring within the first 24 hours. We hypothesize that the full potential of tranexamic acid (TXA) to decrease TKA blood loss has not yet been harnessed because it is rarely used beyond the intraoperative period, and is usually withheld from ‘high-risk’ patients with a history of thromboembolic, cardiovascular, or cerebrovascular disease, a patient group who would benefit greatly from a reduced blood loss.

Aims

A typical pattern of blood loss associated with total hip arthroplasty (THA) is 200 ml intraoperatively and 1.3 l in the first 48 postoperative hours. Tranexamic acid (TXA) is most commonly given as a single preoperative dose only and is often withheld from patients with a history of thromboembolic disease as they are perceived to be “high-risk” with respect to postoperative venous thromboembolism (VTE). The TRanexamic ACid for 24 hours trial (TRAC-24) aimed to identify if an additional 24-hour postoperative TXA regime could further reduce blood loss beyond a once-only dose at the time of surgery, without excluding these high-risk patients.

Aims

Accurate placement of the acetabular component is essential in total hip arthroplasty (THA). The purpose of this study was to determine if the ability to achieve inclination of the acetabular component within the ‘safe-zone’ of 30° to 50° could be improved with the use of an inclinometer.

We report our experience of using a computer navigation system to aid resection of malignant musculoskeletal tumours of the pelvis and limbs and, where appropriate, their subsequent reconstruction. We also highlight circumstances in which navigation should be used with caution.

We resected a musculoskeletal tumour from 18 patients (15 male, three female, mean age of 30 years (13 to 75) using commercially available computer navigation software (Orthomap 3D) and assessed its impact on the accuracy of our surgery. Of nine pelvic tumours, three had a biological reconstruction with extracorporeal irradiation, four underwent endoprosthetic replacement (EPR) and two required no bony reconstruction. There were eight tumours of the bones of the limbs. Four diaphyseal tumours underwent biological reconstruction. Two patients with a sarcoma of the proximal femur and two with a sarcoma of the proximal humerus underwent extra-articular resection and, where appropriate, EPR. One soft-tissue sarcoma of the adductor compartment which involved the femur was resected and reconstructed using an EPR. Computer navigation was used to aid reconstruction in eight patients.

Histological examination of the resected specimens revealed tumour-free margins in all patients. Post-operative radiographs and CT showed that the resection and reconstruction had been carried out as planned in all patients where navigation was used. In two patients, computer navigation had to be abandoned and the operation was completed under CT and radiological control.

The use of computer navigation in musculoskeletal oncology allows accurate identification of the local anatomy and can define the extent of the tumour and proposed resection margins. Furthermore, it helps in reconstruction of limb length, rotation and overall alignment after resection of an appendicular tumour.

Cite this article: Bone Joint J 2015;97-B:258–64.

We performed a case–control study to compare the rates of further surgery, revision and complications, operating time and survival in patients who were treated with either an uncemented hydroxyapatite-coated Corail bipolar femoral stem or a cemented Exeter stem for a displaced intracapsular fracture of the hip. The mean age of the patients in the uncemented group was 82.5 years (53 to 97) and in the cemented group was 82.7 years (51 to 99) We used propensity score matching, adjusting for age, gender and the presence or absence of dementia and comorbidities, to produce a matched cohort receiving an Exeter stem (n = 69) with which to compare the outcome of patients receiving a Corail stem (n = 69). The Corail had a significantly lower all-cause rate of further surgery (p = 0.016; odds ratio (OR) 0.18, 95% CI 0.04 to 0.84) and number of hips undergoing major further surgery (p = 0.029; OR 0.13, 95% CI 0.01 to 1.09). The mean operating time was significantly less for the Corail group than for the cemented Exeter group (59 min [12 to 136] vs 70 min [40 to 175], p = 0.001). The Corail group also had a lower risk of a peri-prosthetic fracture (p = 0.042; OR 0.19, 95% CI 0.01 to 1.42) . There was no difference in the mortality rate between the groups. There were significantly fewer complications in the uncemented group, suggesting that the use of this stem would result in a decreased rate of morbidity in these frail patients. Whether this relates to an improved functional outcome remains unknown.

Cite this article: Bone Joint J 2014;96-B:299–305.

Metal artefact reduction (MAR) MRI is now widely considered to be the standard for imaging metal-on-metal (MoM) hip implants. The Medicines and Healthcare Products Regulatory Agency (MHRA) has recommended cross-sectional imaging for all patients with symptomatic MoM bearings. This paper describes the natural history of MoM disease in a 28 mm MoM total hip replacement (THR) using MAR MRI. Inclusion criteria were patients with MoM THRs who had not been revised and had at least two serial MAR MRI scans. All examinations were reported by an experienced observer and classified as A (normal), B (infection) or C1–C3 (mild, moderate, severe MoM-related abnormalities). Between 2002 and 2011 a total of 239 MRIs were performed on 80 patients (two to four scans per THR); 63 initial MRIs (61%) were normal. On subsequent MRIs, six initially normal scans (9.5%) showed progression to a disease state; 15 (15%) of 103 THRs with sequential scans demonstrated worsening disease on subsequent imaging.

Most patients with a MoM THR who do not undergo early revision have normal MRI scans. Late progression (from normal to abnormal, or from mild to more severe MoM disease) is not common and takes place over several years.

Cite this article: Bone Joint J 2013;95-B:1035–9.

Interest in football continues to increase, with ever younger age groups participating at a competitive level. Football academies have sprung up under the umbrella of professional clubs in an attempt to nurture and develop such talent in a safe manner. However, increased participation predisposes the immature skeleton to injury. Over a five-year period we have prospectively collected data concerning all injuries presenting to the medical team at Newcastle United football academy. We identified 685 injuries in our cohort of 210 players with a mean age of 13.5 years (9 to 18). The majority of injuries (542;79%) were to the lower limb. A total of 20 surgical procedures were performed. Contact injuries accounted for 31% (210) of all injuries and non-contact for 69% (475).The peaks of injury occurred in early September and March. The 15- and 16-year-old age group appeared most at risk, independent of hours of participation. Strategies to minimise injury may be applicable in both the academy setting and the wider general community.

We present the long-term results of pectoralis major transfer to restore elbow flexion in seven patients (ten procedures). The early results in all the patients were encouraging but with longer follow-up a gradual and progressive flexion deformity was observed with a decrease in the arc of flexion in eight elbows, reaching ≥ 90° in all cases.

The results of pectoralis major transfer deteriorate with time due to the development of a recalcitrant flexion deformity of the elbow. With bilateral involvement we now recommend that the procedure be undertaken on one side only to allow the hand to reach the mouth for feeding, while the opposite side remains in extension for perineal toilet.

We performed limb lengthening and correction of deformity of nine long bones of the lower limb in six children (mean age, 14.7 years) with osteogenesis imperfecta (OI). All had femoral lengthening and three also had ipsilateral tibial lengthening. Angular deformities were corrected simultaneously. Five limb segments were treated using a monolateral external fixator and four with the Ilizarov frame. In three children, lengthening was done over previously inserted femoral intramedullary rods.

The mean lengthening achieved was 6.26 cm (mean healing index, 33.25 days/cm). Significant complications included one deep infection, one fracture of the femur and one anterior angulation deformity of the tibia. The abnormal bone of OI tolerated the external fixators throughout the period of lengthening without any episodes of migration of wires or pins through the soft bone. The regenerate bone formed within the time which is normally expected in limb-lengthening procedures performed for other conditions.

We conclude that despite the abnormal bone characteristics, distraction osteogenesis to correct limb-length discrepancy and angular deformity can be performed safely in children with OI.

Localised Langerhans-cell histiocytosis of bone (eosinophilic granuloma) is a benign tumour-like condition with a variable clinical course. Different forms of treatment have been reported to give satisfactory results. However, previous series all contain patients with a wide age range. Our aim was to investigate the effect of skeletal maturity on the rate of recurrence of isolated eosinophilic granuloma of bone excluding those arising in the spine.

We followed up 32 patients with an isolated eosinophilic granuloma for a mean of five years; 17 were skeletally immature. No recurrences were noted in the skeletally immature group even after biopsy alone. By contrast, four of 13 skeletally mature patients had a recurrence and required further surgery. This suggests that eosinophilic granuloma has a low rate of recurrence in skeletally immature patients.

We considered whether a positive margin occurring after resection of a soft-tissue sarcoma of a limb would affect the incidence of local recurrence. Patients with low-grade liposarcomas were expected to be a low-risk group as were those who had positive margins planned before surgery to preserve critical structures. Two groups, however, were expected to be at a higher risk, namely, patients who had undergone unplanned excision elsewhere with a positive margin on re-excision and those with unplanned positive margins occurring during primary resection.

Of 566 patients in a prospective database, 87 with positive margins after limb-sparing surgery and adjuvant radiotherapy were grouped according to the clinical scenario by an observer blinded to the outcome. The rate of local recurrence differed significantly between the two low- (4.2% and 3.6%) and the two high-risk groups (31.6% and 37.5%). This classification therefore provides useful information about the incidence of local recurrence after positive-margin resection.

We describe ten patients with Turner’s syndrome (karyotype 45, XO) who had leg lengthening for short stature. A high incidence of postoperative complications was encountered and many patients required intramedullary fixation as a salvage procedure. We discuss the reasons for this and highlight the differences between our findings and those of a similar series recently reported. In view of the considerable difficulties encountered, we do not recommend leg lengthening in Turner’s syndrome.

We describe a patient who developed avascular necrosis of both humeral trochleae after combination chemotherapy for acute lymphoblastic leukaemia. This presented as progressive stiffness of both elbows with little pain. Radiography and MRI confirmed the presence of avascular necrosis at both sites. This region corresponds to a watershed between the medial and lateral vascular arcades which supply the distal humerus and may explain the susceptibility of this bony region to avascular necrosis. Treatment involved capsulectomy of the elbow and removal of osteophytes giving a good functional outcome on both sides.

We used dual-energy x-ray absorptiometry (DEXA) to evaluate the extent of periprosthetic bone remodelling around a prosthesis for distal femoral reconstruction, the Kotz modular femoral tibial replacement (KMFTR; Howmedica, Rutherford, New Jersey). A total of 23 patients was entered into the study which had four parts: 1) 17 patients were scanned three times on both the implant and contralateral legs to determine whether the precision of DEXA measurements was adequate to estimate bone loss surrounding the anchorage piece of the KMFTR; 2) in 23 patients the bone mineral density (BMD) in different regions of interest surrounding the diaphyseal anchorage was compared with that of the contralateral femur at the same location to test whether there was consistent evidence of loss of BMD adjacent to the prosthetic stem; 3) in 12 patients sequential studies were performed about one year apart to compare bone loss; and 4) bone loss was compared in ten patients with implants fixed by three screws and in 13 without screws.

The mean coefficients of variation (SD/mean) for the 17 sets of repeated scans ranged from 2.9% to 7.8% at different regions of interest in the KMFTR leg and from 1.4% to 2.5% in the contralateral leg. BMD was decreased in the KMFTR leg relative to the contralateral limb and the percentage of BMD loss in general increased as the region of interest moved distally in the femur. Studies done after one year showed no consistent pattern of progressive bone loss between the two measurements. The ten patients with implants fixed by screws were found to have a mean loss of BMD of 42% in the most distal part of the femur, while the 13 without screw fixation had a mean loss of 11%.

DEXA was shown to have adequate precision to evaluate loss of BMD around the KMFTR. This was evident relative to the contralateral leg in all patients and generally increased in the most distal part of the femur. In general, it stabilised between two measurements taken one year apart and was greater surrounding implants fixed by cross-locking screws.

The Sheffield Expanding Intramedullary Rod System was developed after experiencing problems with existing rod systems in the management of osteogenesis imperfecta. Between 1986 and 1996 we treated 74 bones in the lower limb in 28 children at a median follow-up of 5.25 years. We have reviewed 24 children with a total of 60 rods.

Before surgery, all children had had multiple fractures of the lower limb. At review eight patients had experienced no further fractures, but three had suffered five or more subsequently.

Before initial stabilisation, 15 children had never walked, and only three (13%) used walking as their main means of mobility. After surgery, half of those who showed motor arrest were able to walk (p = 0.016). The number of patients able to walk, with or without aids, increased to 17 (p = 0.0001).

We have experienced no evidence of epiphyseal damage after the procedure, and complication rates requiring rod exchange have been low (7%).

We report three complete ruptures and one partial rupture of the flexor pollicis longus tendon in association with the insertion of a volar plate for the treatment of fracture of the distal radius. Rupture was associated with the chronic use of steroids.

We describe the functional results in 14 patients (7 men, 7 women) after subtotal scapulectomy for primary bone and soft-tissue tumours at a specialist musculoskeletal oncology unit. Eight had chondrosarcomas, two Ewing’s sarcomas, one aggressive fibromatosis and three soft-tissue sarcomas. The mean follow-up was 52 months (6 to 120). Analysis of residual symptoms and of range and strength of movement by physicians used the Musculoskeletal Tumour Society rating scale (MSTS). Physical disability was measured by the patients using the Toronto Extremity Salvage Score (TESS).

All 14 patients are still alive, two with systemic disease. Nine had more than 80% of their scapula resected but the glenohumeral joint was preserved in all cases. Eight had full movement and another two achieved 90° of flexion. The mean functional results were good to excellent in all except three patients (mean MSTS = 71.6 and TESS = 79.9). Two of these three patients had considerable pain as a result of brachial neuropathy.

Scapulectomy gives an excellent functional result if the glenohumeral joint is preserved. The rotator cuff could be removed without a severe functional deficit provided that the deltoid was reattached to the scapular remnant and the trapezius.

We studied the quantity and rate of formation of new bone during lengthening of 17 limb segments in 10 patients using dual-energy X-ray absorptiometry (DEXA), ultrasonography and radiography. Whereas new bone was detected by both DEXA and ultrasonography within 1 to 2 weeks of distraction, it was not visible on the radiographs until 4 to 8 weeks. Limb alignment and gap measurement were accurately assessed by DEXA without the need for standard radiographs or scanograms. With ultrasound the distraction gap appeared as an echolucent window which narrowed progressively producing a hyper-reflecting line after which further consolidation could not be assessed. As measured by DEXA the density of the new bone at this stage was approximately 45% of control values and did not represent normal cortication. Whereas ultrasound could be used to identify defects in mineralisation and to determine when to dynamise the fixator system, DEXA could measure the quantity and rate of formation of bone throughout lengthening.

We measured the extent and rate of new bone formation over an 18-month period before, during and after the lengthening of ten leg segments in six patients aged between 8 and 18 years, using dual-energy X-ray absorptiometry (DEXA). New bone formation could be identified within one week of the start of distraction. As lengthening proceeded, the bone density of the gap fell, reaching minimum values at the time of maximal distraction. Consolidation of the regenerating bone was started 1 to 2 weeks later in the tibia, and 2.5 to 3.0 weeks later in the femur. The rate of mineral accretion in new bone was significantly greater in the tibia than in the femur (16 +/- 1.86%/month, and 11 +/- 1.1%/month respectively; mean +/- SEM). There was significant osteoporosis distal to the osteotomy, more in the tibia than in the femur, particularly on the side of the fixator. The bone mineral density of the distal segment remained low at the time of fixator removal (44.2 +/- 5.58% and 61.0 +/- 4.2% of the control values at the tibia and femur respectively) and was only partially reversed by subsequent weight-bearing. We conclude that dual-energy X-ray absorptiometry provides an objective and quantitative assessment of new bone formation during leg lengthening. The technique also allows the measurement of the distraction gap and the assessment of leg alignment from the high-resolution images. Its use may decrease the requirements for conventional radiography.

We report the results of locked Seidel nailing for 30 fractures of the humerus. There were frequent technical difficulties at operation especially with the locking mechanisms. Protrusion of the nail above the greater tuberosity occurred in 12 cases, usually due to inadequate locking, and resulted in shoulder pain and poor function. Poor shoulder function was also seen in five patients with no nail protrusion, presumably because of local rotator cuff damage during insertion. Our results suggest that considerable modifications are required to the nail, and possibly to its site of insertion, before its use can be advocated.

We discuss the role of primary knee arthroplasty in supracondylar and intercondylar fractures of the femur in elderly patients with reference to 13 cases. This method of treatment is shown to be effective and to have good results. It is recommended for all type C and some type A supracondylar fractures in old people.

Five patients with Boyd type II congenital pseudarthrosis of the tibia underwent excision of the pseudarthrosis and double onlay bone grafting. Stability was maintained by extending intramedullary rods. Clinical union was achieved in all cases at a mean of 8.6 months (range six to 11). The rods extended by 15.7% (range 2% to 31.4%) as growth occurred. One rod was removed because of infection and a vascularised free fibular graft was subsequently performed. The extending rods provided stability while union occurred and did not require revision as the legs grew. The rods can be removed easily and have not jeopardized further surgical options.

We made a prospective study of 111 children with acute hip pain to assess whether ultrasound can replace traditional radiography. An effusion was diagnosed in 71% by ultrasound but in only 15% by radiography. This effusion persisted for a mean of nine days; symptoms lasted for five days. Two patients found to have Perthes' disease had longer-lasting effusion and symptoms. Patients without an effusion had no obvious cause for their pain, so the pressure of an effusion from a transient synovitis does not account for all patients with irritable hips. Patients with an effusion persisting for over 24 days (the mean + 2 s.d. of our series) had more symptoms, a significantly larger effusion and greater limitation of movement. They may be more at risk for avascular necrosis. We found that radiographic examination influenced the immediate management of only two patients, those with Perthes' disease. We therefore propose a protocol of management for irritable hip, using ultrasonography at the first presentation of certain categories of patients. This would reduce the number of early radiographs by 75%.

We report the results of using 83 expanding intramedullary rods in 24 children with osteogenesis imperfecta after a mean follow-up of five years three months. In all, 62% of the rods have expanded after one primary operation. Thirty-four additional operations were necessary; 11 for the correction of rotation or angulation deformities and 23 for revision of the rod or T-piece. All these revisions were successful. Complications were more frequent in children who required very small rods. Problems with Bailey-Dubow rods led to the development of the Sheffield rod system; 17 bones treated with these rods are included in the series. Before surgery only eight of the 24 children were able to walk but at review 20 children were walking, 15 without walking aids. Elongating intramedullary rods should be available to all children with osteogenesis imperfecta as they improve walking capability, reduce the number of fractures, prevent deformity and allow integration of the child into society.

We report nine cases of irreducible congenital dislocation of the knee which were treated by early operation with good results. All were resistant to conservative measures and operation was performed at an average age of nine months. The essential abnormality was a short quadriceps muscle together with subluxation of the hamstring muscles to lie anterior to the axis of knee flexion. The quadriceps tendon was lengthened by VY-plasty and in six cases additional length was gained by proximal mobilisation of the muscle. After operation all the patients were able to walk.

After having had a standard decompression for anterior compartment syndrome, five patients presented with persistent symptoms and pressure values above normal. A repeat procedure combined with fasciectomy relieved their pain; postoperative pressure values were normal.

A cortical bone graft on a muscle pedicle was taken from the ulna and transferred to bridge a complete defect of the radius in 16 dogs. In 14 control dogs a free graft was used, that is, one without a muscle pedicle. Union in the group with pedicle grafts was far superior to that in the group with free grafts, mainly because in those with pedicle grafts there was good subperiosteal new bone formation from active viable periosteum. In six of the pedicle grafts the viability of some osteocytes was retained over a 12-week period and in five the graft was almost completely replaced by new bone.

We present three cases of a previously undescribed condition characterised by unilateral tibia vara associated with an area of focal fibrocartilaginous dysplasia in the medial aspect of the proximal tibia. The three children affected were aged 9, 15 and 27 months respectively. Two required tibial osteotomy, but in one the deformity resolved without treatment. The pathogenesis of the focal lesion remains conjectural; the most likely explanation is that the mesenchymal anlage of the tibial metaphysis has, for unknown reasons, developed abnormally at the insertion of the pes anserinus.

Seven pectoralis major transfers in children suffering from bilateral paralysis of elbow flexion due to arthrogryposis or to trauma are reported. A technique is described in which the muscle is mobilised from the clavicle to allow the tendon of insertion to be attached to the biceps tendon at the elbow. The biceps tendon was found to be present and could be mobilised forwards in all the arthrogrypotic elbows. Subjectively, the results were considered by patients or parents to be very good in six cases and fair in one. Elbow flexion power against gravity and against some resistance was achieved in all patients except one. The overall function was very good in one elbow, good in two, fair in three and poor in only one. The merits of the various procedures described for the restoration of elbow flexion in arthrogryposis are discussed. It is concluded that total pectoralis major transfer by the method described here has given the best results.

Excellent results can be achieved by plating fractures of the shaft of the humerus in patients with multiple injuries. This helps in nursing care and in the management of other injuries. In 38 patients admitted to a regional trauma centre, 39 humeral shaft fractures were plated. There were 27 men and 11 women, with an average age of 31.5 years. Fourteen of the humeral fractures were compound and 20 had significant comminution; 23 were fixed by a plate on the day of admission and all 39 by the twentieth day. Follow-up of 34 fractures showed that all had united, 33 primarily. All patients but one had a fully functional shoulder and no patient with a fractured humerus alone had lost any elbow movement. Complications were rare--one case each of non-union, fixation failure and infection. No permanent nerve injuries were produced at operation. The plating of fractures of the humerus in these circumstances has been shown to produce excellent results and has a place in the management of the patient with multiple injuries.

We report the ulnar impingement syndrome, which is caused by a shortened ulna impinging on the distal radius and causing a painful, disabling pseudarthrosis. Of the 11 cases reported, 10 were due to excision of the distal ulna after injury to the wrist; the other was a result of a growth arrest after a fracture of the distal ulna in a child. The symptoms are a painful, clicking wrist and a weak grip; clinical examination reveals a narrow wrist with pain on compression of the radius and ulna and on forced supination. Radiographs in the majority of cases show scalloping of the distal radius corresponding to the site of impingement. The mechanism by which ulnar impingement occurs after radio-ulnar convergence is illustrated. The plan of management for the young patient with traumatic dysfunction of the distal radio-ulnar joint is discussed; excision of the lower end of the ulna is not advised in such patients.

Ruptures of the calcaneal tendon which present late may be repaired using carbon fibre to induce a neotendon. The operative technique is described and the results of five cases reviewed. The average muscle power obtained was 88% of normal, and the thickness of the neotendon was 148% of that of the normal side. It would appear that this tendon formation in man is comparable to that previously described in sheep.

There are differences of opinion about the pathogenesis of Perthes' disease. All are agreed that it is due to ischaemia, but the cause of this and the size and number of infarctions are in dispute. Through the generosity of the contributors six whole femoral heads and core biopsies of five other cases have been studied radiographically and histologically. The findings ranged from an ischaemic arrest of ossification in the capital articular cartilage without infarction to multiple complete infarctions of the epiphysial bone. The ensuing reparative process contributes to the pathology, which is of a range to warrant grading or grouping.

1. Twenty-one cases of congenital dislocation of the hip were found on examination of 1,881 consecutive neonates on the first day of life, giving an incidence of eleven per 1,000 live births.

2. Insignificant high-pitched "clicks" were noted in 10 per cent of newborn children.

3. Conversion of half of the patients with hip dislocation to normal occurred during the first post-natal week.

4. Joint laxity was not a feature of the newborn with congenital dislocation of the hip.

5. Oestradiol, oestrone and oestriol were estimated in twenty-fourhour urine samples collected from sixteen patients with congenital dislocation of the hip and nineteen matched controls during the first six days of life. No significant differences in oestrogen output between the two groups were found.

6. The hypothesis that congenital dislocation of the hip is a result of an inborn error of oestrogen metabolism in the newborn is not supported.

1. Six cases of malunited anterior Monteggia fracture have been treated, five of them successfully, by open reduction and reconstruction of the orbicular ligament by turning down a slip from the triceps tendon.

2. One relapse occurred after a slight injury; this was because of an unsuitable triceps tendon.

3. A slip from the triceps tendon has retained reduction of the head of the radius in a patient with congenital dislocation of the radial head.

One of the aims of this work was to find criteria by which the quality of bone as a supporting tissue might be judged. This inevitably involves discussion and, if possible, assessment, of the relative importance of the inorganic and organic material of the bone. It is relatively easy to measure the mineral content, and for that reason it has always received more than its due share of attention.

In the present experiment the composition of the ash of all bones was remarkably constant, with a Ca/P ratio of 2. Furthermore, X-ray crystallography showed that the structure of the inorganic material was the same in all cases. The great difficulty of measuring variations in the quality of the organic material which is, of course, protein in nature makes it impossible to say how much it influences bone strength. Since at least 40 per cent. of the bone is collagen, either a quantitative or a qualitative alteration might alter bone strength. X-ray crystallography revealed no qualitative differences in the collagen material of bones of the three groups; so that for the present it would seem safer to assume that alterations in the physical properties of the bones are due to variations in the relative proportions of organic and inorganic constituents (Dawson 1946, Bell et al. 1947).

These experiments show that the three diets produce highly significant differences in the percentage of ash, in S_B, and in E. It is possible that some variations in the percentage of ash are due to variations in the absolute collagen (weight of collagen in unit volume of bone substance); but the range of variation in the percentage of ash leaves no reasonable doubt that differences in percentage ash between the diet groups are due essentially to differences in absolute ash. Presumably the collagen contributes something to the strength of the bone; but the indications are that it plays a minor part and that the relative weakness and flexibility of rachitic bones is due to decrease in the absolute ash content. Within any one diet group, the relation between percentage ash and the other two variables, S_B and E, is masked by other sources of variation such as those associated with the many measurements involved; and thus the correlation between percentage ash and S_B, and also between percentage ash and E, is not significant.

At first sight, the scatter diagrams (Figs. 5 and 6) appear to indicate a correlation between ash and S_B, and between ash and E. Closer inspection shows, however, that the apparent trend is due largely to differences between the means of the diet groups, and that the points within any one group show no such obvious trend. Figure 7 shows that the position with regard to correlation between S_B and E is very different. Here there is an obvious trend within each diet group; the amount of scatter is very much less. Calculation shows that, even when the differences between the means of diet groups is excluded, there is still a significant correlation between S_B and E. The question of the correlation between the three variables is discussed more fully in the addendum to this paper.

Although the "goodness" of a bone is usually judged by its breaking stress, the experimental findings recorded above suggest that it may be assessed equally well on the basis of elastic properties as shown by Young's modulus. Normal bones, group S in these experiments, were elastic up to 79 per cent. of their breaking stress (Table II): the poorer bones of groups R and N were, however, only a little inferior in this respect. In some cases there was no apparent deviation of the load-deflexion curve from a straight line until the bone was about to break. Such a curve was published in the first paper of this series (Bell, Cuthbertson and Orr 1941), but in the light of further experience this curve is scarcely typical. The terminal falling over of the curve is illustrated in Figure 4 and is much more marked in the bones of group R.

While stress at the upper limit of elasticity varies over a wide range in the three groups (Table II and Fig. 4), the strain at this point is remarkably constant at about 1·5 per cent. This same percentage displacement must occur between the molecules of the bone material at the elastic limit—and it may be that, up to this amount of molecular displacement, the deformation is reversible; but that beyond it, plastic changes occur. We have no evidence as to whether the limiting displacement concerns mineral or protein constituents of the bone, or both.

We have already commented on the remarkable strength of bone material (Bell et al., 1941). The breaking stress of normal rat bone is about the same as that of cast iron, and about half that of mild steel. Young's modulus, however, is only one-tenth that of cast iron and one-twentieth that of steel. Thus bone, despite its lightness (specific gravity about 2·5 as compared with 7·9 for iron), is remarkably strong and at the same time more flexible than might be expected. Presumably the biological advantage is that greater flexibility helps to absorb sudden impacts. It is unusual in metallic substances to find the elastic modulus proportional to the strength; this is more characteristic of materials like concrete and timber. Another remarkable property of bone is that it remains elastic up to three-quarters of the breaking stress. Most metals show considerable ductility before reaching their breaking point.

While Young's modulus is of interest, both on its own account and as an index of the quality of the bone, its close association with breaking stress suggests that it might be used to predict the maximum load which a bone can carry safely. Since E, unlike S_B, can be measured without damage, useful information might be gained by measuring the elasticity of living human bones.