The rational strategies for detecting developmental dysplasia of the hip at the age of 4-6 months old infants: a prospective study

Using ultrasound in evaluation of infant's hip development can reduce surgical procedures, hospitalization and late presentation of developmental dysplasia of the hip (DDH). The increasing incidence of DDH after ultrasound examination is observed and published by many authors. In a prospective study, radiograph of every single ultrasonographic positive hip in infants older than three months, was taken and analyzed in order to see whether it affects infants splintage rate in treating DDH. In a period of 30 months, clinical and simple static ultrasonographic examinations according to Graf were performed on 1430 consecutive infant hips in patients aged between 4 and 6 months. Sonographic positive hips were radiographed and acetabular index (AI) values on simple AP radiographs were analyzed. The sonographic DDH incidence was 51.75 per 1000 hips (51.75 per thousand). After X-ray examination of all 74 ultrasonographic positive hips, only 44 remained abnormal and required treatment indicating a true DDH incidence of 30.77 per 1000 hips (30.77 per thousand). The difference in incidence per ultrasonographic and X-ray positive hips is statistically significant p < 0.01 (t = 5,536). The rational approach in detection of DDH in a child more than 3 months old is to do radiographic assessment of every sonographic positive hip.     

Hip contact stress during normal and staircase walking: the influence of acetabular anteversion angle and lateral coverage of the acetabulum

Hip contact stress is considered to be an important biomechanical factor related to development of coxarthrosis. The effect of the lateral coverage of the acetabulum on the hip contact stress has been demonstrated in several studies of hip dysplasia, whereas the effect of the anterior anteversion remains unclear. Therefore, the joint hip contact stress during normal level walking and staircase walking, in normal and dysplastic hips, for small and large acetabular anteversion angle was computed. For small acetabular anteversion angle, the hip contact stress is slightly increased (less than 15%) in staircase walking when compared with normal walking. In hips with large angle of acetabular anteversion, walking downstairs significantly increases the maximal peak contact stress (70% in normal hips and 115% in dysplastic hips) whereas walking upstairs decreases the peak contact stress (4% in normal hips and 34% in dysplastic hips) in comparison to normal walking. Based on the presented results, we suggest that the acetabular anteversion should be considered in biomechanical evaluation of the hips, especially when the lateral coverage of the acetabulum is small.     

Development of acetabulum after closed reduction in developmental hip dysplasia

The radiographic study of acetabulum development after closed reduction of 87 congenitally dislocated hips in 64 patients who were treated between 1980 and 1990 was conducted retrospectively. An average patient's age at the moment of closed hip reduction was 16 months (range between 6 to 24 months). On the average, patients were 16 months old (range between 6 to 24 months) at the moment of closed hip reduction. Development of bony acetabulum was followed for 8 years by measuring the AC angle. According to the age at the moment of closed reduction patients were sorted into three groups, the first group from 6 months to 12 months old (39 hips), the second group from 12 to 24 months old (28 hips) and the third group from 24 to 36 months old (20 hips). At the beginning of the treatment mean values of the AC angle in the first group were 37.2 degrees +/- 3.31 degrees; in the second group 39.7 degrees +/- 3.2 degrees; and in the third group 43.8 degrees +/- 2.7 degrees. The final values of the AC angle in the first group were 14.7 degrees +/- 3.21 degrees, in the second group 21 degrees +/- 3.02 degrees; and in the third group 24 degrees +/- 3.76 degrees. Normally and mildly dysplastic hips were achieved in 80.5% treated hips, while in 19.5% treated hips medium and serious dysplasia retarded. The acetabulum development in dysplastic hip after retained stable concentric reduction was the same as in the normal hip. The operative reconstruction of acetabulum should not be done before the third year of life.     

基于三维CT的模拟手术在大龄DDH患儿外科治疗中的应用研究

目的:通过对大龄发育性髋关节脱位(developmental dysplasia of the hip,DDH)患儿进行术前模拟手术,实现术中精确截骨及旋转角度,从而达到个体化治疗,改善患儿预后的目的。方法:本研究按照术前规划方式分为两组,一组为传统手术组;另一组为模拟手术组。共20例患儿均采用骨盆三联截骨术+股骨截骨术治疗,传统手术组10例,模拟手术组10例,手术时平均年龄为11.3岁,平均随访时间24.2个月。所有患儿均于术前行骨盆三维重建CT检查,测量CE角、股骨前倾角及髋臼指数,在mimics软件中,模拟手术方案,确定术中股骨截骨需要旋转的角度及骨盆截骨的位置,术中按照模拟手术的结果进行操作。术前评价指标使用Tonnis分级,术后评价指标使用改进的Trevor评分系统。结果:模拟手术组Tonnis分级3级4髋,Tonnis分级4级8髋;传统手术组Tonnis分级3级4髋,Tonnis分级4级9髋,两组患儿术前严重程度无显著性差异。依据Trevor评分,模拟手术组8髋(67%)优秀,3髋(25%)良好,1髋(8%)一般。传统手术组5髋(38%)优秀,5髋(38%)良好,3髋(23%)一般。两组有显著性差异。并发症:术后传统手术组3例患儿有不同程度的股骨头坏死。结论:大龄DDH患儿术前模拟手术,可以达到术中精确截骨及旋转角度,可改善患儿预后,实现该类患者的个体化治疗。     

The advancement in the early diagnostics of developmental hip dysplasia in infants--the role of ultrasound screening

Developmental hip dysplasia (DDH) is a deformation of the locomotor system that can occur on previously normal bone structures in late fetal development. The study was undertaken to determine the value of continuous ultrasound screening in early diagnosis of DDH. In the area covered by the Labin Primary Health Center, clinical and ultrasound examination of the hips was performed in all children during the fourth month of life from 1989 to 2001. The incidence of DDH was 3.3%. The authors compared the obtained results with results of clinically examined hips in the period from 1968 to 1988 (incidence 1.7%), before the ultrasound diagnostics was introduced. The hip radiograms were performed only in children with clinically positive signs of higher degree of DHD. The ultrasound examination was performed with Hitachi EUB 410 real time linear transducer with a 5 MHz probe. The examiners used methods and criteria suggested by Graf. The most useful feature of the ultrasound method is that it allows the visualization of cartilaginous femoral head and its relationship to the acetabulum in infants. The method is non-invasive, repeatable, without risk of ionizing radiation or need for the contrast material administration. The results of the study revealed an important increase of DHD incidence after the introduction of ultrasound examination. US is highly sensitive in hip imaging and reveals the existence of a number of dislocated and subluxated hips that otherwise would be missed. The anomaly was discovered in the early period of life, which is very important for appropriate therapeutic effects.     

How should dysplastic human hips be evaluated?

Dysplasia of the human hip is characterised by insufficient anterolateral covering of femoral head by the acetabulum. In our study, we evaluated dysplastic human hip joints using biomechanical parameters (the peak contact stress in the weight-bearing area of the hip - pmax) and X-ray image parameters (the centre-edge angle - thetaCE, the transverse acetabular inclination angle - thetaUS, the acetabular index of the weight-bearing zone - thetaAC, the ACM angle - thetaACM, and the hip value - HV). The purpose of this study is to make use of X-ray and biomechanical parameters to evaluate hips diagnosed with "hip dysplasia", and to establish whether or not there is a correlation between the two. Our results show a statistically significant correlation between pmax and thetaCE, thetaUS and thetaAC. The correlation between pmax and thetaACM and HV is not statistically significant.     

Validation of the ‘FeMorph’ software in planning cam osteochondroplasty by incorporating labral morphology

Impingement resulting from a cam deformity may cause pain, limit the hip joint range of motion (RoM) and lead to osteoarthritis. We have previously developed FeMorph software to quantify and plan corrective surgery and predict hip RoM post surgery. This study aimed to validate the software and evaluate the influence of the acetabular labrum on hip RoM. Computed tomography data from 92 femur-pelvis pairs were analysed in conjunction with the inter/intra-observer reliability. Four cadaveric hips were dissected, and the three-dimensional (3D) shape and size of the acetabular labrum for these hips was obtained using laser scan. The influence of the acetabular labrum in the RoM and subsequent planning for corrective surgery were then evaluated in cadavers for models with and without a labrum, and used as a first step towards validation of FeMorph RoM prediction. FeMorph was successfully used to model cam deformities and plan corrective surgery. Three-dimensional alpha angles were reduced to below 50° after virtual surgery without an excessive reduction in femoral neck cross-sectional area, which could increase fracture risk. A mean increase of 8° ± 2° in permitted internal rotation was observed during impingement testing following removal of the labrum. FeMorph provides a reliable and useful method to model and plan cam deformity correction. This study indicates that the presence of the labrum is responsible for a substantial decrease in permitted internal rotation at the hip joint. This has implications for surgical planning models which often only account for bony impingement.     

Problems in the Early Recognition of Congenital Hip Dislocation

Seven cases of infants whose hips were clinically normal at birth, and in whom hip dislocation was later recognized, are reported. In spite of the widespread practice of routine examination of the hips of newborn babies, infants and young children are still presenting with congenital dislocation of the hip. This is probably owing to there being two aetiological types. Every opportunity to re-examine the hips of children under 2 or even 3 years of age must be taken and a high level of suspicion maintained.     

The composition,structure and stability of a group II chaperonin are temperature regulated in a hyperthermophilic archaeon

The hyperthermoacidophilic archaeon Sulfolobus shibatae contains group II chaperonins, known as rosettasomes, which are two nine-membered rings composed of three different 60 kDa subunits (TF55 alpha, beta and gamma). We sequenced the gene for the gamma subunit and studied the temperature-dependent changes in alpha, beta and gamma expression, their association into rosettasomes and their phylogenetic relationships. Alpha and beta gene expression was increased by heat shock (30 min, 86 degrees C) and decreased by cold shock (30 min, 60 degrees C). Gamma expression was undetectable at heat shock temperatures and low at normal temperatures (75-79 degrees C), but induced by cold shock. Polyacrylamide gel electrophoresis indicated that in vitro alpha and beta subunits form homo-oligomeric rosettasomes, and mixtures of alpha, beta and gamma form hetero-oligomeric rosettasomes. Transmission electron microscopy revealed that beta homo-oligomeric rosettasomes and all hetero-oligomeric rosettasomes associate into filaments. In vivo rosettasomes were hetero-oligomeric with an average subunit ratio of 1alpha:1beta:0.1gamma in cultures grown at 75 degrees C, a ratio of 1alpha:3beta:1gamma in cultures grown at 60 degrees C and a ratio of 2alpha:3beta:0gamma after 86 degrees C heat shock. Using differential scanning calorimetry, we determined denaturation temperatures (Tm) for alpha, beta and gamma subunits of 95.7 degrees C, 96.7 degrees C and 80.5 degrees C, respectively, and observed that rosettasomes containing gamma were relatively less stable than those with alpha and/or beta only. We propose that, in vivo, the rosettasome structure is determined by the relative abundance of subunits and not by a fixed geometry. Furthermore, phylogenetic analyses indicate that archaeal chaperonin subunits underwent multiple duplication events within species (paralogy). The independent evolution of these paralogues raises the possibility that chaperonins have functionally diversified between species.     

Molecular dynamics simulations of a monofucosylated biantennary glycan of the N-acetyllactosamine type: the human lactotransferrin glycan.

Molecular dynamics simulations were carried out to explore the conformational flexibility of the antennae of N-linked glycans. They were performed over 200 ps in vacuo on the complete disialylated monofucosylated biantennary glycan of the N-acetyllactosaminic type. Starting from a bird-conformation, the 3-D-structure evolved through 9 successive transitional states to a new, compact and energetically favorable conformation which had never been previously described. In this conformation, both antennae are organized in two coplanar loops rolled in a contrary direction and oriented perpendicularly to the plane of the di-N-acetyl chitobiose residue leading to a 'lobster conformation'. All the glycosidic linkages of the disialylated monofucosylated biantennary glycan, except the Fuc(alpha 1-6)GlcNAc(beta 1-), were modified by a phase transition. Particularly, the Man(beta 1-4) GlcNAc(beta 1-) linkage, which was previously described by NMR and X-ray diffraction as a rigid one, was involved in numerous conformational changes during 83 ps, even before the first transition phase. The freedom of mobility of the torsional angles of the Man(alpha 1-6)Man(beta 1-) linkage was limited, under these simulation conditions, to the angle psi which took three values: 30 degrees, 90 degrees and 180 degrees. Moreover, from 150 ps up to the end of the simulation, the value of the torsional angle omega of the NeuAc(alpha 2-6)Gal(beta 1-) linkage of the alpha-1,6-antenna continuously swung between 60 degrees and -60 degrees. Finally, we observed that the values of the torsional angles of the three linkages: NeuAc(alpha 2-6)Gal(beta 1-), Gal(beta 1-4)GlcNAc(beta 1-) and GlcNAc(beta 1-2)Man(beta 1-) of each of the two antennae were different, demonstrating their asymmetric conformation.     

Acetabular morphology and resurfacing design

The bony surfaces of 18 archaeological hemipelves were scanned using a 3D laser surface scanner and CyDir™ software on a Silicon Graphics workstation. The acetabular area was selected and point data from the approximately spherical bone surface saved. These data were input to a MATLAB routine that calculated the radius and centre of the best-fit sphere. The goodness of fit was estimated using the mean and standard deviation of the distance of the bone surface points from the sphere surface. Eight points, at approximately equal distances around the acetabular rim, were selected with reference to bony landmarks. A plane containing three of these points served as an orientation reference plane. The vectors joining the eight rim points to the centre of the best-fit sphere were found. The angles between these vectors and the normal to the reference plane were calculated. Paired angles were summed to give the angle subtended by the acetabular rim in four directions. The overall mean angle was 158° (range of mean angles 145°–173°). The largest individual angles, some exceeding 180°, were in the superior–inferior direction, while the mean angle in the anterior–posterior direction, i.e. that controlling flexion-extension, was 152°. Males had larger subtended angles than females, although the difference was not statistically significant. Simulated reaming increased all angles by approximately 10°. The subtended angles are important parameters in the design of the acetabular component of a hip replacement and particularly important in resurfacing hip replacement when the volume available is tightly constrained.     

A novel acetabular alignment guide for THR using selective anatomic landmarks on the pelvis

This paper presents a novel approach for acetabular alignment during the implant of a prosthetic hip joint in a natural pelvis. The alignment instrument uses selective anatomic bony landmarks on the pelvis, which are accessible in surgery, to guide the placement of the acetabular component in the appropriate orientation. A closed form solution, involving both a forward and reverse analysis, is presented to relate the parameters of the device with the abduction and anteversion angles. Using mathematical models, this device should allow the surgeon to place the acetabular component with an orientation between 10.9 degrees and 19.1 degrees anteversion and 35.7 degrees and 44.3 degrees abduction with 95% confidence in a male/left specimen for the commonly accepted target of 15 degrees anteversion and 40 degrees abduction. This device is currently being used successfully by one of the authors in THR surgery.     

Cooperativity in the thermosome

The thermosome from Thermoplasma acidophilum is a type II chaperonin composed of eight alpha and eight beta subunits. The genes encoding the two types of subunit were co-expressed in Escherichia coli and the alpha8/beta8 complex purified from the cell extract. The isolated complex showed steady-state ATPase properties characteristic of the thermosome purified from the native organism and was capable of enhancing the folding yield of a thermostable enzyme at elevated temperature (55 degrees C). To compare the nucleotide response of this double-ring structure with the type I and more compositionally heterogeneous type II chaperonins, the tryptophan residue within the alpha subunit was used as a fluorescence reporter of the conformational changes within the thermosome induced by the binding of nucleotides. Stopped-flow measurements of indole fluorescence at 55 degrees C showed that there is a fast (approximately 350 s(-1)) and a slow (approximately 0.6 s(-1)) structural rearrangement when ATP binds to the thermosome. Further examination of the fast rearrangement showed that the associated rate constant followed a two-phase saturation profile, as it does for GroEL and for the type II chaperonin from the eukaryotic cytoplasm. This result, in keeping with these precedents, reveals that the thermosome is also a negatively cooperative system with respect to inter-ring communications, i.e. the first ring loads with higher affinity than the second. As in the case of GroEL, the loading of the second ring is weakened by ADP, implying that asymmetric ATP/ADP complexes are favoured over symmetric ones. Despite the difference in co-protein involvement in the type I and II chaperonins, these observations show that negative cooperativity is a common feature of all chaperonins thus far examined. This property results in a strong preference for asymmetry in nucleotide occupancy and implies at least some commonality with the reciprocating encapsulation mechanism shown for the GroE chaperonins.     

Effect of acetabular component anteversion on dislocation mechanisms in total hip arthroplasty

Quantifying soft-tissue tension around the hip joint during total hip arthroplasty remains difficult. In this study, a three-dimensional computer-aided design model was developed to clarify how component position in total hip arthroplasty contributes to the primary cause of posterior dislocation in cases of flexion, adduction and internal rotation. To better understand the influences of anteversion angle of the acetabular component, its effects on the primary causes of dislocations and the range of motion were investigated. Three different primary dislocation mechanisms were noted: impingement of the prosthetic femoral neck on the cup liner; impingement of the osseous femur on the osseous pelvis; and spontaneous dislocation caused by soft-tissue traction without impingement. Spontaneous dislocation could be detected by calculating hip forces at any thigh position using the computer-aided design model developed. In computer analysis, a transition from prosthetic impingement rate to osseous impingement rate occurred with increasing anteversion angle of the acetabular component. Spontaneous dislocation was detected at angles > 10° of anteversion of the acetabular component when flexion occurred with extreme adduction and internal rotation. This study demonstrated the possibility of spontaneous dislocation that results not from prosthetic or bony impingement but from muscle traction with increased range of motion.     

Prognostic value of refined Wiberg's angle in hip development

There are many angles measured on hip radiographs, but their prognostic value for hip development in patient with DDH is controversial. Our aim was to find out how the development of congenitally luxated hip could be predicted after reduction using Wiberg's CE angle and CE angle refined according to Ogata. 119 hips of 61 patients were retrospectively evaluated after reduction from 3 months of age until at least full osteomaturity. According to results of CE angles measurements good and poor hip development group were formed. Healthy hips of the patients with unilateral DDH were considered to be the control group. Negative linear trend of the curve connecting Wiberg's CE angle values measured within first postreductive years is a poor prognostic sign of hip development as are absolute negative values of refined CE angles. In cases where negative initial refined CE angles are combined with negative linear trend of Wiberg's CE angles prognosis is extremely poor. Both Wiberg's and Ogata's CE angle are of prognostic value regarding hip development and should be measured when evaluating AP radiographs of infant's hip.     

Retroperitoneal hematoma with bone resorption around the acetabular component after total hip arthroplasty: a case report and review of the literature

ABSTRACT: INTRODUCTION: Vascular complications related to cup-fixating screws penetrating the medial acetabular wall during total hip arthroplasty are not uncommon but rarely are associated with serious adverse events in the late post-operative period. CASE PRESENTATION: We present the case of a 77-year-old Japanese woman who developed progressive extensive bone resorption and large hematoma in the acetabulum 13 years after total hip arthroplasty. On admission to our hospital, she was on oral warfarin (1.5mg/day) for atrial fibrillation. About 5 months after the initiation of anticoagulant therapy, she suffered a major fall followed by massive subcutaneous and pelvic girdle bleeding, predominantly on the medial side of the right thigh, but a fracture or damage of total hip arthroplasty was not evident on an emergency orthopedic evaluation. One year after the accident, a routine follow-up examination showed an asymptomatic osteolytic lesion in the acetabulum on the right pelvis, and 2 years later our patient noticed progressive pain in her right hip during walking. A large osteolytic lesion was noted in the right acetabulum on a plain radiograph. On high-resolution computed tomography and magnetic resonance imaging, a huge granulomatous lesion in the acetabulum was suggestive of chronic hematoma in intrapelvic and extrapelvic gluteal regions. A closer computed tomography examination showed that one of the screws used for fixation of the acetabular component in the total hip arthroplasty had penetrated the acetabular bone and had reached the pelvic cavity. Surgery was performed in a single session by means of two approaches: anterior midline transperitoneal address to resect the low-density mass lesion followed by posterolateral acetabular implant re-settlement. CONCLUSIONS: Though rare, total hip arthroplasty-related late vascular complications could be serious and potentially affect the limb and quality of life.     

Bone geometry of the hip is associated with obesity and early structural damage – a 3.0 T magnetic resonance imaging study of community-based adults

IntroductionThe mechanism by which obesity increases the risk of hip osteoarthritis is unclear. One possibility may be by mediating abnormalities in bony geometry, which may in turn be associated with early structural abnormalities, such as cartilage defects and bone marrow lesions.MethodsOne hundred and forty one older adults with no diagnosed hip osteoarthritis had weight and body mass index measured between 1990 and 1994 and again in 2009 to 2010. Acetabular depth and lateral centre edge angle, both measures of acetabular over-coverage, as well as femoral head cartilage volume, cartilage defects and bone marrow lesions were assessed with 3.0 T magnetic resonance imaging performed in 2009 to 2010.ResultsCurrent body mass index, weight and weight gain were associated with increased acetabular depth and lateral centre edge angle (all P ≤ 0.01). For every 1 mm increase in acetabular depth, femoral head cartilage volume reduced by 59 mm³ (95% confidence interval (CI) 20 mm³ to 98 mm³, P < 0.01). Greater acetabular depth was associated with an increased risk of cartilage defects (odds ratio (OR) 1.22, 95% CI 1.03 to 1.44, P = 0.02) and bone marrow lesions (OR 1.29, 95% CI 1.01 to 1.64, P = 0.04) in the central region of the femoral head. Lateral centre edge angle was not associated with hip structure.ConclusionsObesity is associated with acetabular over-coverage. Increased acetabular depth, but not the lateral centre edge angle, is associated with reduced femoral head cartilage volume and an increased risk of cartilage defects and bone marrow lesions. Minimising any deepening of the acetabulum (for example, through weight management) might help to reduce the incidence of hip osteoarthritis.     

Ceramic cups for hip endoprostheses. 6: Cup design, inclination and antetorsion angle modify range of motion and impingement]

The present investigation focuses on total hip replacement using ceramic acetabular components. The relationship between the position of the cup and the range of motion (ROM) was investigated. A limited range of motion may cause impingement, which is defined as contact between the femoral neck and the rim of the acetabular cup. Impingement may result in wear, chipping, fracture or dislocation of the femoral head. Joint movements were simulated in a three-dimensional CAD program. The results obtained underscore the importance of correct positioning and design of the cup for achieving a ROM as close to the physiological situation as possible. With ceramic cups, the inclination angle should not be more than 45 degrees, and the antetorsion angle between 10 and 15 degrees. If the cup is too vertical, the risk of dislocation and fracture of the ceramic increases. If, on the other hand, the angle of inclination is too small, flexion and abduction will be greatly limited. The study shows that acetabular components with non-recessed ceramic inserts should not be used. Slight recession of the insert helps to avoid impingement. The ROM is reduced and the risk of impingement appreciably increased when mushroom-shaped femoral heads (XL heads) or ceramic inserts protected by a polyethylene ring are used.     

Role of the acetabular labrum in load support across the hip joint

The relatively high incidence of labral tears among patients presenting with hip pain suggests that the acetabular labrum is often subjected to injurious loading in vivo. However, it is unclear whether the labrum participates in load transfer across the joint during activities of daily living. This study examined the role of the acetabular labrum in load transfer for hips with normal acetabular geometry and acetabular dysplasia using subject-specific finite element analysis. Models were generated from volumetric CT data and analyzed with and without the labrum during activities of daily living. The labrum in the dysplastic model supported 4-11% of the total load transferred across the joint, while the labrum in the normal model supported only 1-2% of the total load. Despite the increased load transferred to the acetabular cartilage in simulations without the labrum, there were minimal differences in cartilage contact stresses. This was because the load supported by the cartilage correlated with the cartilage contact area. A higher percentage of load was transferred to the labrum in the dysplastic model because the femoral head achieved equilibrium near the lateral edge of the acetabulum. The results of this study suggest that the labrum plays a larger role in load transfer and joint stability in hips with acetabular dysplasia than in hips with normal acetabular geometry.     

Synthesis and polymorphism of 3-acyl-sn-glycerols

3-Acyl-sn-glycerols with even-numbered saturated fatty acyl chains from decanoate to lignocerate were synthesized. Successful hydrolysis of the long acyl chain intermediate 1,2-isopropylidene-3-acyl-sn-glycerols from stearate to lignocerate was accomplished by applying the compounds to silica gel and exposing them to hydrogen chloride gas at -75 degrees C. The purity of the compounds was checked by boric acid impregnated thin-layer chromatography, 13C NMR, and reverse-phase high-pressure liquid chromatography. Differential scanning calorimetry and X-ray diffraction techniques were used to study the polymorphism of the compounds. In the beta phase obtained from solvent of crystallization, the acyl chain packing was in a two-dimensional oblique lattice with specific chain-chain interactions with a tilt angle of 55.4 degrees from the bilayer plane. The thickness of the region containing two glycerol head groups was 12.7 A. The phase transition enthalpy of melting for the beta phase was 1.06 kcal/mol of CH2. On being cooled these compounds crystallized reversibly to an unstable alpha phase, which on being further cooled underwent a second crystallization to a beta or beta' phase. The thermodynamic parameters and long spacings of these compounds in both beta and alpha phases were linear, indicating isostructural packing in each phase. The enthalpy of the melting transition of the alpha phase was 0.69 kcal/mol of CH2. In this phase, the chains were packed in a hexagonal lattice with nonspecific chain-chain interactions. The thickness of the head-group region (12.2 A) and the tilt angle (55 degrees) of the acyl chains in the alpha phase were very similar to those in the beta phase.