The Hip – from Cradle to Grave

From a dissertation read before the Society on Friday, 26th November, 1965 From birth to old age the hip joint provides a fascinating spectrum of disease processes; this provides a useful starting point for a study of the interaction of development of structure with development of function and the pathological variants of this interaction. Because of limitations of space only a few disease processes will be chosen and much of the review will be devoted to normal structure and function. Copyright Royal Medical Society. All rights reserved. The copyright is retained by the author and the Royal Medical Society, except where explicitly otherwise stated. Scans have been produced by the Digital Imaging Unit at Edinburgh University Library. Res Medica is supported by the University of Edinburgh’s Journal Hosting Service: http://journals.ed.ac.uk ISSN: 2051-7580 (Online) ISSN: 0482-3206 (Print) Res Medica is published by the Royal Medical Society, 5/5 Bristo Square, Edinburgh, EH8 9AL Res Medica, April 1967, 5(4): 13-15, 17-18 doi: 10.2218/resmedica.v5i4.495 THE HIP FROM CRADLE TO GRAVE Ian R. Williams. M.B.. Ch.B. From a dissertation read before the Society on Friday, 26th November, 1965 From birth to old age the hip joint provides a fascinating spectrum of disease processes; this provides a useful starting point for a study of the interaction of development of structure with developm ent of function and the patho­ logical variants of this interaction. Because of lim itations of space only a few disease processes will be chosen and much of the review will be devoted to normal structure and function.


THE HIP -FROM CRADLE TO G R A V E
Ian R. Williams.M.B.. Ch.B.
From a dissertation read before the Society on Friday, 26th Novem ber, 1965 From birth to old age the hip joint provides a fascinating spectrum of disease processes; this provides a useful starting point for a study of the interaction of developm ent of structure with developm ent of function and the patho logical variants o f this interaction.Because of lim itations of space only a few disease processes will be chosen and m uch of the review w ill be devoted to normal structure and function.

A N A T O M Y
T h e general anatom y of the adult hip joint is well know n and w ill therefore only be briefly m entioned.It is a synovial ball and socket joint in which the alm ost spherical head of the fem ur sits in the deep cup of the acetabulum , the two apposed surfaces being alm ost congruent.T h e neck of the fem ur is angulated m edially on the shaft at an angle o f 13 5 ° and is anteverted 2 5 0 at birth.
T h e anatom y o f the blood supply is im port ant both during developm ent and later life.T h e arterial supply is from two m ain sources in the adult; the artery' of the ligam entum teres and the re tinacular vessels w hich ascend the neck of the fem ur in the fibres of the joint capsule, entering the synovial cavity at the upper part of the neck.T h e relative im portance o f these vessels varies greatly during developm ent; the follow ing table attem pts to show w hich vessels are responsible for supply at each age:-Retinacular vessels -Posterolateral -Superior M etaphyseal; Lateral Epiphyseal.A fter m aturity has been reached all the vessels take part in the supply but the relative im portance o f each is disputed; the artery of th e ligam entum teres has been said to be capable of supplying the w hole head (W ald en strom 19 34, W o lco tt 1943.)Strange (1965) believes that this would be possible in only 2 0 % o f the population; in a further 2 0 % he suggests that it has no function and in 6 0 % its supply is restricted to 10 % of the head.L ik e wise the contribution of th e N u trient artery of the fem ur has been variously assessed: T u cker (1949) and T ru eta and H arrison (19 53) do not believe that it has any role in supplying the head.
Racial variations occur especially with respect to the im portance of the artery' of the ligam entum teres in the negroid, the supply being m uch greater in the 4 -9 years period.
T h e im portance of these facts should becom e more apparent on considering pathological processes later in the review.

PHYSIOLOGY
T h e articular surfaces of the joint are of hyaline cartilage w hich, in spite o f its name, has strands o f collagen running in a basket work fashion, serving to prevent lateral expansion as a result of vertical com pression.T h e load on these surfaces is high and thus perfect distri bution is required, as is a low coefficient of friction.T h e coefficient o f friction has been calculated by m any people w ith results ranging from 0.0075 to 0.023; the average of C h arn ley's experim ents was 0 .013.T h is com pares with 0.03 for ice on ice and 0 .1 for m echanical ball bearings.
In spite o f this low coefficient o f friction the m ovem ent of the surfaces, one over the other, produces continual abrasion of surface layers -this m ay be considered as wear and tear or sim ply as replacem ent; the result appears to be the same.T h e other im portant effect which m ovem ent has, is the com pression and expan sion of the cartilage and this m ay be im portant in lubrication, the synovial fluid entering and leaving the cartilage as it w ould a sponge.T h e flow o f fluid in and out o f the cartilage m ay also be im portant to the nutrition of the avascular tissue.
T h e lubrication has been thought to occur in m any different ways; a com bination of several w ould seem to be possible.A hydrodynam ic m ethod in w hich one surface floats on a film o f fluid on the other surface is the sim plest, but T an n er ( 1 9 59) believed that the nature of the fluid precludes this under the conditions existing in the hip joint.A refine m ent of this has been suggested b y C h arnley (1959) who favours a form of boundary lubrication; in this form the lubricant flu id has an affinity for both surfaces enabling a thin film of fluid to be present even at high pres sures.A further refinem ent com bines all that has so far been described w ith the expression of the fluid from the cartilage under pressure T h e viscosity o f the fluid itself m ay be important in that the extent of polym erisation i reduced in several diseases o f the hip join (M atsunaga, Ioki and A oki 1958).
D epoly m erisation is associated w ith decreased viscosity M any of the disease processes of the hip involve physical displacem ent of one form 0 another and hence a brief consideration o f the forces acting on the joint is essential.W h e r a person is standing on both legs the w eight or each joint is half the body w eight excluding the legs, which would be about 72 lbs. in a 12 stone m an.T h is would act vertically through the head of the fem ur.W h e n w alking, however, the w eight is borne on one leg at a tim e leading to enorm ous forces acting at the hip joint; in the figure this can easily be seen.
T h e centre o f gravity passes slightly to the same side as the weight bearing hip; this leads to an adduction m om ent W x P C .T o balance this the abductors, gluteus m edius and m ini mus, m ust exert an equal m om ent and because their line of action is m uch nearer the centre, P, the force m ust be correspondingly greater.In fact P C is at least twice P B (Inm an 19 53 gave 2.4 -2.6) so that the m uscles m ust achieve a force of at least tw ice th e body w eight; this means that the total load on the hip, the resultant, is alm ost three times the body weight, acting downwards and 20° outwards.
B alance in th e antero posterior direction is controlled by gluteus maximus.
T h e aetiology and pathogenesis o f several disease processes w ill now be considered in order to illustrate the im portance o f som e of the above factors.

CONGENITAL DISLOCATION OF THE HIP
T h is disorder occurs in 15 per 10,000 live births in this country, bu t in countries such as Japan, N orth Italy and B rittan y the incidence is as high as 40 per 10,000 live births.T h e incidence is m uch low er in countries where m others carry' their babies astride their backs.G irls are affected four times m ore frequently than boys and the left hip three times more than th e right; it is bilateral in only 2 5 % of cases.D islocation at birth and dislocation as part of a general disorder, e.g.athrogryphosis, are not considered here.
T h e typical dislocation is not a primary developmental defect because the joint develops by cleavage of a solid block of mesoderm which is complete by the tenth week of intrauterine life.
Among the many factors suggested are here ditary and familial influences but it is not clear whether this represents a genetic predisposition to dislocation as such or merely a tendency to predisposing factors such as placental site.G ill in 1948 suggested a disturbance of the maternal environment; perhaps after the discovery of the effects of Thalidom ide this may seem a possi bility.Dennis Brown (1948) postulated intra uterine mechanical stress as a cause; pressure of the uterus on a flexed knee in a breech present ation forcing the head down and back out of the acetabulum.In 1962 Barlow produced results to suggest that the incidence was increased in breech presentation but it was not possible to say which was cause and which effect.Acetabular hypoplasia and increased anteversion of the femoral neck have both been suggested as primary factors but both are more likely to be secondary.V on Rosen showed, in 1962, that in babies with C .D .H .the pubic symphysis could be distracted twice the distance of that of a normal child; he also found an increased ex e rtio n of O estrogens in the urine of these babies, for the first three days of life.Hence it seems possible that hormonal factors may relax ligaments which allows the mechanical stresses in the uterus to produce subluxation or dislocation and increased anteversion.Consequent upon this the limbus, consisting of cartilage and a fold of capsule, turns into the joint and this together with hypertrophy of the ligamentum teres and capsule leads to difficulty in reduction.W ith delayed reduction hypoplasia of the acetabulum becomes apparent and the inturned limbus may calcify (Somerville 1953).After a good early reduction almost normal structure and function will be achieved.In a variable proportion of patients an epiphysitis develops probably due to interference with the blood supply to the ossific nucleus of the normal side.T he patho genesis is not clear but the long intracartilagenous course of the vessels would render them liable to compression in the Frog or Bachelor positions used for reduction.

PERTHES' DISEASE
Between the ages of 3 and 16, but especially 4 and 10 years, children are at risk to Perthes' disease; it is 2 or 3 times more common in boys and bilateral in about 10 % of eases.It is more common than C .D .H .but its aetiology and treatment arc subjects of great mystery.Hereditary and familial influences have been suggested but are not generally thought to be important; likewise infection, trauma and hormonal factors have been suggested.T he most acceptable theories implicate vascular disturbances leading to an avascular necrosis, although Ratliff (1962) compared the process w ith that follow ing fracture o f the neck of the fem ur and said that they were not the same.
A t this stage in life the epiphysis is dependent on the lateral epiphyseal vessels alone for its blood supply; com pression of these vessels w ould lead to necrosis.T h is is supported by the fact that the process is m ost com m on at the anterior margin which is furthest from the blood supply, and that it does not occur in negroes, w ho have a supply from the ligam entum teres (Tructa 1959).
Experim ental findings are m ainly extrapol ation from anim als because o f the innocent nature of the condition; in general these support a loss of arterial supply as the primary' event.T h is is follow ed by m ultiple fractures of the trabeculae, but because of the avascularity no reaction occurs except in the synovial m embrane -sw elling of this m em brane and the ligam entum teres leading to lateral displacem ent of the head.If fibrosis occurs at this stage the displacem ent will be m ore marked and prolonged.T h e inflam m atory response causes a decalcification in the area but the ossific nucleus, having no blood supply, docs not take part, giving an appearance of increased radio density of the head.
From surviving tissue the repair begins and thus at this stage m ay b e found areas of necrotic bone and of new bone; because some superficial subchondral layers of bone are supplied w ith nutrients through the surviving cartilage it is also possible to find areas of new bone overlying necrotic bone.
T h e pain caused by the inflam m atory re action around the join t causes the patient to adopt a lim p in which the centre of gravity m oves laterally decreasing the work dem anded of gluteus medius and thus the load on the hip joint.In fact the load in this position is only the body weight and acts vertically through the fem oral head; this in turn means that the load is spread over a m uch sm aller area of the head, increasing the pressure.T h is im m ense pressure is associated with indentation of the head at every step causing trabecular fractures.D uring repair th e pressure and the enlarged soft tissue lead to flattening o f the head followed by flattening of the acetabulum if treatm ent is not prom pt and effective.SLIPPED EPIPHYSIS D isplacem ent of the upper fem oral epiphysis is a not uncom m on problem affecting boys m ore than girls between the ages of 12 and 15 years.A t this tim e the adult sex horm ones are just beginning to be secreted at a higher level and this m ay be im portant.T h e ratio o f the G row th H orm one and the G onadotrophic H orm ones appears to control the rate and duration o f grow th, w ith a fall o f G .H .and increase in sex horm ones occurring through puberty.A delay or fall in the production of sex horm ones by the anterior pituitary which later returns to norm al or continued high out put of G .H . both give rise to a relative increase in the am ount of G .H .T h e form er gives a mild Froh lich 's syndrom e and the latter a m ild pituitary gigantism .
In 19 50 Harris showed th e effect of this to be a widening of the layer of cartilage cells in the epiphyseal line which is a structurally weaker zone and is rendered even weaker.Scott (1956) showed that 7 0 % of the patients had Frohlich-like features and 30 % had signs of G .H .excess, in his series of patients.
T h e weakened epiphysis is then exposed to the stresses o f w eight bearing and o f progres sion: th e tendency to leave the epiphysis with the pelvis is increased.T h e pain so caused produces a lim p in which the weight is trans ferred laterally and acts vertically on the head of the fem ur as described above.T h is change in the direction o f the force greatly increases the shearing strain at the epiphyseal line, and the fem ur becom es m ore vertical, again increas ing the strain; these changes act to displace the epiphysis downwards and backwards.
T h e displacem ent is not sudden b u t gradual usually, and in 2 5 % of cases becom es bilateral.

OSTEOARTHRITIS
Osteoarthritis is the m ajor disease o f the lim p in adult life and m ay be roughly divided into prim ary and secondary; the aetiology of the prim ary form is rather a m ystery, m ost attem pts to explain it using a secondary theory.
A normal joint can survive norm al work for a hundred years, whereas an abnorm al joint w ill show changes at an earlier age.W o lff's L aw may b e considered alongside this observation: just as norm al function gives rise to norm al structure, abnorm al function m ay be expected to give rise to abnorm al structure; thus osteo arthritis m ay represent a change in structure related to a lim p (abnormal function) from som e prim ary pathological change.
In the gluteus m edius lim p, described above, the w eight of the body is seen to act on a much reduced area of the fem oral head, so that in spite of reduced load there are areas o f very high pressure and areas of very lo w pressure.T h is loss of the norm al com pression and expansion of cartilage results in decreased nutritional supply, first in the deeper layers, and this undernourished tissue disrupts under high pressure giving rise to fibrillation and erosion.T h is allows a further sh ift o f the head, accentuating th e pressure differences and lead ing to dam age to the bony articular surface.T h e trabeculae undergo changes to counteract the changes, becom ing thickened; this requires an increase in vascularisation in a wedge shaped vascular supply area which becom es apparent radiographically.In spite of strengthening, trabecular infractions still occur and local hyperaem ia denotes the start of repair pro cesses.A t th e sam e tim e som e trabeculae lose their blood supply, as in all fractures, and syn ovial flu id enters the necrotic areas giving rise to th e cyst form ation seen on X-ray pictures.W ith a loss of fine structure com es a loss of gross structure and flattening of the head, proportional to the extent of devascularisation.T h e fluid trapped in the bone transm its pressure to the cyst walls causing a reactive scherasis.
Strange suggests that the size o f the head and acetabulum now increases in an attem pt to reduce the pressure; this is the process of lip pin g and osteophyte form ation w hich H arri son et al (19 5 3) considered to be prim arily due to increased vascularisation.H arrison et al considered osteoarthritis to be prim arily a vas cular reaction to degeneration of articular cartilage and that abnorm alities resulting, e.g.osteophytes, are evidence o f persistent attem pt at repair.
T h e theories presented above are really all involving a secondary m echanism though a prim ary disease process m ay not always be obvious.