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James Gill and Majeed Shakokani
GT
P
I
R
S
O
I
G
Nerve to
piriformis
(S1-S2)
Figure 4.3 Anatomy of the short external rotators. GT = greater
trochanter, PIRI = piriformis, SG = superior gemellus, OI = obturator
internus, IG = inferior gemellus and QF = quadratus femoris
Nerve to
obturator
internus
I
G
Nerve to
quadratus
femoris
I
Q
F
Figure 4.4 Sciatic nerve variations:
a. Sciatic nerve exits beneath piriformis (84.2%)
b. Peroneal nerve exits through piriformis and tibial nerve exits
beneath piriformis (11.7%)
c. Peroneal nerve exits above piriformis and tibial nerve exits
beneath piriformis (3.3%)
d. Sciatic nerve exits through piriformis (0.8%)
e. Common peroneal exits above piriformis and tibial nerve exits
through piriformis (hypothetical)
f. Sciatic nerve exits above piriformis (hypothetical)
sciatic foramen. Obturator internus exits the lesser
sciatic foramen, not the greater sciatic foramen.
The pudendal nerve and the nerve to obturator
internus both exit the greater sciatic foramen and
re-enter the lesser sciatic foramen. Obturator
externus does not pass through either the lesser
or greater sciatic foramen. Obturator externus originates from ischiopubic ramus and the obturator
membrane which spans the obturator foramen.
16. Answer D. Obturator internus
Obturator internus exits rather than enters the
pelvis via the lesser sciatic foramen. The pudendal
nerve, internal pudendal vessels (artery and vein)
and the nerve to obturator internus all enter the
pelvis via the lesser sciatic foramen having first
exited the pelvis via the greater sciatic foramen.
17. Answer C. Common peroneal division passing
through piriformis and tibial nerve division
exiting beneath piriformis
Beaton and Anson performed a cadaveric study of
the anatomical variants of the relationship of the
sciatic nerve to the piriformis muscle. The most
prevalent relationship is the sciatic nerve exiting
the greater sciatic nerve beneath the piriformis
muscle and occurs in about 84% of the population. The next most common variation is the
sciatic nerve dividing in the pelvis with the
common peroneal division passing through piriformis and the tibial division exiting beneath piriformis; this variant occurs in approximately 12%
of the population (Figure 4.4).
Beaton LE, Anson BJ. The relation of the
sciatic nerve and of its subdivisions to the piriformis muscle. Anatom Rec. 1937;70:1–5.
18. Answer D. Short head of biceps femoris
Weakness of ankle dorsiflexion and foot eversion
represent a palsy of the superficial peroneal nerve.
Intact ankle plantar flexion would suggest the tibial
nerveis spared. The short head ofbiceps isthe most
proximal muscle to be supplied by the common
peroneal nerve and therefore should be the first to
be re-innervated. The long head of biceps is supplied by the tibial nerve. Two other muscles in the
leg also have dual nerve supply: adductor magnus
and pectineus. The adductor part of adductor
magnus is supplied by the posterior division of
the obturator nerve, whereas the hamstrings portion is supplied by the tibial nerve. The anterior
fibres of the pectineus are supplied by the femoral
54
Hip II Structured SBA
nerve, whereas the posterior fibres are supplied by
the anterior division of the obturator nerve.
Peroneus longus is supplied by the superficial
branch of the peroneal nerve, tibialis posterior
and popliteus by the tibialis nerve.
19. Answer A. Bone scintigraphy
Three-phase bone scintigraphy is the most sensitive
imaging modality for early detection of heterotopic
ossification. Plain film radiographs might not show
heterotopic ossification until 1–4 weeks after it is
visible on bone scintigraphy. Single-photon emission computed tomography (SPECT) improves the
sensitivity and specificity of planar bone scintigraphy due to more accurate localisation of activity.
Ghanem MA, Dannoon S, Elgazzar AH. The
added value of SPECT-CT in the detection of
heterotopic ossification on bone scintigraphy.
Skeletal Radiol. 2020;49:291–298.
20. Answer B. Cotyloid fossa, superior aspect of the
obturator foramen and the cortical surface of
the true pelvis
The pelvic teardrop is a radiographic landmark
seen in the anteroposterior view of the pelvis. The
teardrop is formed by a continuous U-shaped surface of bone, the lateral border of which is made up
of the cortical surface of the cotyloid fossa (true
floor of the acetabulum). This surface of bone
continues through the acetabular notch and curves
inferior medially beneath the transverse acetabular
ligament through the superior margin of the
obturator foramen. This forms the inferior bend
of the U of the teardrop. The medial aspect of the
teardrop is made of the cortical surface in the true
pelvis. The bony structures that form the pelvic
teardrop were confirmed in a radiographic study
in which a strip of lead foil was applied to the true
floor of the acetabulum, through the obturator
foramen and onto the cortical surface of the true
pelvis. The teardrop is a frequently used landmark
in total hip arthroplasty. When templating for a
THA, the inferior part of the cup should be level
with the bottom of the teardrop.
Vare VB, Jr. The anatomy of the pelvic tear
figure. J Bone Joint Surg Am. 1952;34-A:167–169.
21. Answer A. Ankylosing spondylitis
Ankylosing spondylitis is associated with
increased acetabular anteversion. Slipped upper
femoral epiphysis (SUFE), Legg–Calvé–Perthes
disease and pincer femoral acetabular impingement (FAI) are characteristically associated with
acetabular retroversion. Rheumatoid arthritis is
associated with acetabular protrusion. The
increased acetabular anteversion associated with
ankylosing spondylitis predisposes patients with
ankylosing spondylitis who are undergoing total
hip arthroplasty to anterior dislocation. Patients
with ankylosing spondylitis compensate for fixed
kyphosis of the spine with pelvic extension,
which leads to increased acetabular anteversion.
Pelvic extension and knee flexion allow a patient
with fixed kyphosis of the spine (positive sagittal
balance) to bring the head back over the pelvis.
Addressing spinal deformity in patients with
ankylosing spondylitis prior to total hip arthroplasty has been advocated.
Direito-Santos B et al. Acetabular retrover-
sion: diagnosis and treatment. I Open Rev.
2018;3:595–603.
22. Answer D. Femoral head medial to ilioischial
line
The definition of protrusion is the femoral head
protruding medial to the ilioischial line. The
iliopectineal line is medial to the ilioischial line,
so if the femoral head protrudes medial to the
iliopectineal line there will be acetab ular protrusion. However, this is not the defining threshold
of protrusion. An acetabular fossa medial to the
ilioischial line defines coxa profunda (deep acetabular socket). Other definitions for acetabular
protrusion have been proposed:
Centre-edge angle greater than 40°and
medialisation of the medial wall of the
acetabulum protruding past the ilioischial line.
Acetabular fossa greater than 3mm beyond
the ilioischial line in men and greater than
6mm in women.
23. Answer E. Superior lateral femoral neck stress
fracture involving 25% of neck width
All tension side (superior femoral neck) stress
fractures require internal fixation. Compression
(inferior femoral neck) stress fractures involving
more than 50% width of the femoral neck also
need internal fixation. The mainstay of treatment
for piriformis syndrome and iliotibial band
55
James Gill and Majeed Shakokani
Figure 4.5
Iliofemoral ligament
syndrome is physical therapy. Symptomatic cam
lesions may be amenable to surgical intervention
when conservative measures have been exhausted.
24. Answer D. Material that exhibits linear stress
strain relationship until the point of failure
This describes a material that is brittle. Ceramic
bearing surfaces fracture because they are brittle
and have low toughness and poor fracture resistance. The area under the stress – strain curve
describes a material’s toughness. Ductile materials exhibit non-linear change in length (plastic
deformation) beyond the elastic limit on a stress–
strain curve. Failure at a point below the ultimate
tensile strength secondary to repetitive loading is
termed ‘fatigue failure’. Progressive deformation
in response to a constant force over a prolonged
period of time is the definition of ‘creep’.
26. Answer A. Iliofemoral
The iliofemoral ligament, also known as the
Y-ligament of Bigelow, is the strongest ligament in
the body. It originates from the anterior inferior
iliac spine and then fans out in an inverted Y shape
to attach along the anterior intertrochanteric line
of the femur (Figure 4.5). The ligament is a static
stabiliser and checks hip extension. Because the
ligament limits hip extension, it allows maintenance
of an upright posture with slight hip extension.
With reduced need for muscle contractions, the
ligament is therefore energy conserving.
27. Answer B. Cup inclination 40°, cup anteversion
15°, femoral stem anteversion 15°
In a classic paper, Lewinnek et al. (1978) proposed
arelative‘safety zone’ for the acetabular cup position
of 40±10° inclination and 15±10° anteversion.
The study has a number of limitations; it was a
case series of 300 total hip arthroplasties but there
were only 9 dislocations and there was no mention
of femoral stem anteversion. Cup version cannot be
considered in isolation; the relationship between
cup and stem version is described by the term
‘combined anteversion’. While all surgeons accept
the importance of implant orientation, there are
numerous other factors that are important for hip
stability, which are commonly grouped into surgical
factors (implant position, surgical approach, surgeon
experience/volume), implant factors (head size,
head–neck ratio, type of cup/acetabular liner) and
patient factors (muscle weakness, soft tissue quality).
Lewinnek GE, Lewis JL, Tarr R, Compere
CL, Zimmerman JR. Dislocations after total hip-
replacement arthroplasties. J Bone Joint Surg Am.
1978;60:217–220.
25. Answer A. Early prosthetic loosening
Early prosthetic loosening is associated with
sickle cell disease; the mechanism is extended
bone infarct disease. Psoriasis is associated with
higher periprosthetic infection rate. Paget’s disease is associated with increased blood loss, and
ankylosing spondylitis is associated with a higher
risk of heterotopic ossification.
Kenanidis E, Kapriniotis K, Anagnostis P,
Potoupnis M, Christofilopoulos P, Tsiridis E.
Total hip arthroplasty in sickle cell disease: a
systematic review. EFORT Open Rev. 2020 Mar
2;5(3):180–188.
56
28. Answer C. 35°
The combined anteversion is the sum total of
acetabular version and femoral anteversion.
Combined anteversion can be assessed on the
operating table once components have been
implanted and before the joint is closed by internally rotating the femur with the knee flexed until
the cup and base of the femoral head are coplanar; the angle made between the lower leg
and the floor is the combined anteversion. The
optimal combined version is debated. Dorr et al.
(2009) believe there is a wide safe zone and recommended aiming for 25–45° with a mean of 35°.
r
Hip II Structured SBA
Dorr LD, Malik A, Dastane M, Zhinian W.
Combined anteversion technique for total hip
arthroplasty. Clin Orthop Rel Res.
2009;467:119–127.
29. Answer C. Gluteus medius and minimus
deficiency
A constrained polyethylene liner should be
reserved for recurrent desolators with soft tissue
dysfunction. A constrained liner encircles the
femoral head and mechanically prevents the head
from displacing out of the socket. Constrained
liners increase the stress at the bone – implant
interface, which increases the risk of early
loosening. In all of the other options listed, suboptimal implant position or design could be corrected with revision of implants.
30. Answer D. 5cm
Numerous sources report that the superior gluteal nerve may be damaged if the gluteus medius
is split more than 5cm proxim al to the greater
trochanter. However, it may lie even closer. In a
cadaveric study of 44 hips, the superior gluteal
nerve was found to be a mean of 4.8cm (range
2–9cm) from the greater trochanter. Ramesh et
al. (1996) reported 11% risk of superior gluteal
nerve denervation following a series of Hard inge
approaches in which the gluteus medius was not
splint more than 4cm from the greater
trochanter.
Khan T, Knowles D. Damage to the superior
gluteal nerve during the direct lateral approach
to the hip: a cadaveric study. J Arthroplasty
2007;22:1198–1200.
Ramesh M et al. Damage to the superior
gluteal nerve after the Hardinge approach to the
hip. J Bone Surg Br. 1996;78:903–906.
31. Answer B. Lateral to the prima ry compressive
trabeculae and medial to the secondary compressive trabeculae
Ward’s triangle is a space formed near the centre
of the femoral neck by the intersection of three
trabecular bundles, namely, the principal compressive, the secondary compressive and the tensile trabecular (Figure 4.6). This central region,
containing some thin and loosely arranged trabeculae, defines a neutral axis where tensile and
compressive forces balance each other. The three
boundaries of Ward’s triangle are medially the
primary compressive trabeculae, laterally the secondary compressive trabeculae and superiorly
the primary tensile trabeculae. Ward’s triangle
itself is not a sign of osteoporosis; however,
expansion of Ward’s triangle, which can be visualised on a plain film AP hip radiograph, is due
to loss of trabeculae.
Principal
compressive group
Principal
tensile group
W = Ward's triangle
Secondary
compressive group
Figure 4.6 Ward’s triangle
Greater trochante
group
W
Secondary
tensile group
57
James Gill and Majeed Shakokani
32. Answer A. Ascending branch of the lateral fem-
oral circumflex artery
The ascendi ng branch of the lateral femoral circumflex artery crosses the interval proximally
between sartorius (femoral nerve) and tensor
fascia lata (superior gluteal nerve). The ascending
branch of the lateral femoral circumflex artery
should be identified and ligated to prevent excessive bleeding. In 1919 Smith-Petersen first
described a direct anterior approach (DAA) to
the hip for reducing congen ital hip dislocations.
Smith-Petersen is also credited with the first
DAA for hip arthroplasty in 1949. Over the subsequent decades, several modifications to his
technique have occurred, along with the development of new instruments to make it less invasive
and easier to perform for THA.
33. Answer B. 0.5µm
Particulates in the range of 0.1–1.0μm (submicron) are biologically active, with those in the
range of 0.1–0.5μm are thought to be the most
biologically active and responsible for osteolysis.
Macrophages phagocytose polyethylene particles,
as they are a similar size to bacteria. Activated
macrophages release cytokines, which stimulate
osteoblasts to release RANK ligand. This in turn
leads to activation of osteoclasts, which resorb
bone. The cytokines released by activated macrophages are TNFα, IL-1, IL-6, PGE2 and PDGF.
Activated macrophages also directly absorb bone
via the release of matrix metalloproteinases
(MMPs) and upregulate osteoclast differentiation
via macrophage-colony stimulating factor
(MCSF) (Figure 4.7).
34. Answer E. Total hip arthroplasty
A subchondral lucent line describes the crescent
sign. The crescent sign features in both the
modified Ficat and Steinberg staging systems
for adult hip osteonecrosis. Presence of the crescent sign indicates imminent femoral head collapse. Joint preserving procedures such as core
decompression, osteotomy and vascularised bone
grafting are limited to pre-collapse femoral
heads, whereas joint replacement procedures
are indicated in the presence of femoral head
collapse.
35. Answer C. Osteoprotegerin
Osteoprotegerin is a decoy receptor of the receptor activator of nuclear factor kappa-B ligand
(RANKL). Osteoprotegerin binds RANKL to
limit its activity. Platelet-derived growth factor,
interleukin-1 and interleukin-6 are involved in
signalling from macrophages to activate osteoclasts to resorb bone. Osteoclasts are activated
indirectly via osteoblasts and the RANKL
pathway.
36. Answer D. Uncemented, fully porous coated
titanium stem
A number of factors contribute to stress
shielding: stem fixation; cemented composite
58
Osteoblasts
Macrophage
IL1
–
+
PGE2
+
TNF
+
Monocyte osteoclast precursors
+
M-CSF
MMP
Osteoclast
Figure 4.7 A series of proinflammatory
factors, including IL-1, IL-6, PGE2, TNF-α,
can be produced by wear particleactivated macrophages. These cytokines
can induce the expression of RANKL,
which activates osteoclasts. Macrophages
also release matrix metalloproteinases
(MMPs) and downregulate production of
tissue inhibitors of metal metalloproteinases (TIMPs)
Hip II Structured SBA
beam versus taper slip, uncemented fully porous
coated versus proximally porous coated, Young’s
modulus of the cement stem and radius of the
stem (Figure 4.8). With taper slip stem design,
viscoelastic properties of bone cement allow slip
of the stem in the cement mantle as it is loaded
axially. Axial forces are converted into hoop
stresses throughout the length of the taper; therefore, provided the cement mantle is not excessively thick, bone is loaded al ong the length of
the implant. Composite beam cemented stem
designs are so named because the stem, cement
and bone behave as one (a composite) construct.
With composite beam stem designs, forces are
concentrated at the distal tip of the stem opposed
to throughout the proximal femur with taper slip
designs. Composite beam designs therefore lead
to more proximal bone loss than do taper slip
designs. Calcar flanges were added to composite
beam designs in an attempt to load the bony
calcar and prevent proximal stress shielding.
The loading of the femur is similar in uncemented stems to composite beam cemented stems.
Proximal stress shielding is reduced in proximally porous coated stems opposed to fully porous
coated stems. Increasing stem radius increases
the stiffness and therefore increases stress
shielding. The following commonly used metals
for femoral stems are listed in decreasing order
of stiffness: cobalt-chrome, stainless steel and
titanium. Answer D is correct as it combines
the greatest number of factors which contribute
to stress shielding.
37. Answer C. Adductor longus and gracilis
There is no internervous plane for the medial
approach to the hip. The superficial intermuscular plane is between adductor longus and gracilis,
both of which are innervated by the anterior
division of the obturator nerve. The deep intermuscular plane is between adductor brevis supplied by the anterior division of the obturator
nerve and adductor magnus, which has dual
innervation. The adductor portion is supplied
by the posterior division of the obturator nerve,
and the hamstrings portion is supplied by the
tibial portion of the sciatic nerve.
38. Answer A. Adductor brevis – posterior division
of obturator nerve
Gracilis is supplied by the anterior division of the
obturator nerve. Two muscles around the hip
and one muscle in the thigh have dual innervation: adductor magnus, pectineus and biceps
femoris. Adductor magnus is supplied by the
posterior division of the obturator nerve and
the tibial nerve. Pectineus is supplied by the
femoral and the obturator nerve. Biceps femoris
is supplied by the tibial nerve (long head) and
peroneal nerve (short head). Adductor longus is
supplied by the anterior division of the obturator
nerve.
39. Answer B. Iliopsoas
The medial and lateral circumflex arteries are
named according to their relationship to the
iliopsoas tendon (Figure 4.9). The medial
Figure 4.8 Anteroposterior
(AP) radiograph of right reverse
hybrid THA, uncemented fully
porous coated stem, with
significant proximal femur
bone loss secondary to stress
shielding and osteolysis
Medial circumex artery
Capsule
Ascendant
cervical
arteries
Iliopsoas
Femoral
artery
Figure 4.9 Femoral neck blood supply
Femoral neck
Lateral circumex artery
Greater
trochanter
Ascendant
branches
59
James Gill and Majeed Shakokani
femoral circumflex artery (MFCA) is a branch of
the profu ndal femoris; it winds around the
medial side of the femur, passing between pectineus and iliopsoas (medial to it). The femoral
head receives its blood supply primarily from the
deep branch of the MFCA. The medial and lateral femoral circumflex arteries contribute to an
extracapsular arterial ring at the base of the femoral neck. The extracapsular arterial ring gives
rise to the retinacular vessels, which run superiorly along the femoral neck until they reach the
cartilaginous border of the femoral head, at
which point they penetrate the femoral head.
The lateral femoral circumflex artery is also usually a branch of the profundal femoris artery and
passes lateral to the iliopsoas tendon. The calcar
femorale refers to the vertical plate of dense
cancellous bone that develops in the posterior
femoral neck separated from the lesser trochanter. It is not to be confused with the calcar, which
is the lowermost and thickest point of the cortex
constituting the medial wall of the femoral neck.
Hammer A. The calcar femorale: a new per-
spective. J Orthop Surg (Hong Kong)
2019;27:2309499019848778.
40. Answer B. Flexible femoral stem
Flexible femoral stems are not optimal for stem
fixation, as they place increased stress on the
cement mantle; hence, most femoral stems are
made from stainless steel (316L) or cobaltchrome. A cement mantle >2mm reduces the
risk of cement mantle fracture. Stem centralisation using a plastic tip centraliser aims to produce an even cement mantle around the femoral
stem, which decreases stress on the cement
mantle and improves fixation. The centraliser is
hollow to allow the stem to subside by leaving a
space for the tip of the stem to subside into. Prior
to the use of hollow stem centralisers, ‘punch out’
fractures were observed in the distal portion of
the cement mantle. Highly polished femoral
stems reduce shear stresses at the cemen t– stem
interface and allow subsidence, which converts
axial load into radial hoop stresses. Matt finish
taper slip stems are thought to have failed early
due to abrasive wear at the cement–stem interface. Vacuum preparation of cement is a feature
of third generation cementation and reduces the
porosity of the cement, which reduces stress
points in the cement and increases the strength
of stem fixation.
41. Answer B. The capsule attaches more distally on
the neck posteriorly compared with anteriorly
The capsule attaches more distally on the femoral
neck anteriorly compared with posteriorly.
Posteriorly, the capsule attaches more proximally. Hence, when performing arthroplasty for
subcapital femoral neck fracture, there are often
fragments of the head attached to the capsule
posteriorly once the head has been removed.
The gluteus minimus inserts into the anterior
capsule and greater trochanter. The reflected
head of the rectus femoris originates from the
anterior hip joint capsule, whereas the direct
head originates from the anterior inferior iliac
spine. The ischiofemoral ligament forms the posterior hip joint capsule and is divided when performing the Southern–Moore (posterior)
approach to the hip joint. The iliofemoral ligament forms the anterior hip joint capsule and is
divided when entering the hip joint using the
Smith-Petersen (anterior) approach. The iliofemoral ligament and the direct head of the rectus
femoris tendon share an attachment to the ilium
just superior to the acetabulum.
42. Answer B. L2
Radicular pain from nerve roots L1 and L2 may
mimic referred hip pain in the groin. L2 radiculopathy may mimic referred hip pain in the
thigh. Impingement of the lateral cutaneous
nerve of the thigh causes numbness and burning
pain over the lateral aspect of the thigh, the
condition known as meralgia paresthetica.
Causes of compression include tight belts or
irritation from seat belts worn during prolo nged
driving, pregnancy and obesity. The posterior
cutaneous nerve of the thigh is rarely a cause of
complaint due to impingement. It arises from the
S1 to S3 nerve roots, so sensation may be diminished as part of cauda equina syndrome. The
obturator nerve supplies sensation to the medial
thigh and groin. The femoral nerve supplies sensation to the anterior medial thigh extending
distal to the area supplied by the obturator nerve.
43. Answer A. Anterior intrasubstance labral tear
and a poster ior acetabular cart ilage lesion
60
Hip II Structured SBA
A pincer-type lesion is due to overcoverage of the
acetabulum and is common in middle-aged
females. This results in abnormal contact
between the acetabular rim and the femoral
head–neck junction. The anterior superior femoral head–neck junction is levered against the
acetabular rim and a contrecoup cartilage lesion
may occur on the posterior inferior acetabulum.
Protrusio and acetabular retroversion are causes
of acetabular overcoverage. Acetabular retroversion may be seen on a plain film AP radiograph
of the pelvis; the anterior wall may be seen crossing lateral to the posterior wall. This is called the
crossover sign. The other common pattern of
femoral acetabular impingement is termed cam
impingement. Cam impingement is most
common in young males and is caused by a
non-spherical femoral head and decreased
head–neck offset. In hip flexion the aspherical
head engages with acetabular cartilage, causing
a shearing force that results in delamination of
the anterior acetabular cartilage and avulsion of
the anterior labrum. The radiographic appearance of a non-spherical femoral head that may
cause cam-type impingement is described as a
‘pistol grip deformity’.
Imam S, Khanduja V. Current concepts in the
diagnosis and management of femoroacetabular
impingement. Int Orthop. 2011;35:1427–1435.
44. Answer B. 2.3mm, 3.3mm
If the femoral neck angle is 125° this amounts to
a 35° angle from the horizontal. Therefore, any
further increase in the neck length will result in a
smaller increase in the leg length compared with
offset. When considering the addition of a ‘plus
4mm’ head, answer E (4.0mm, 0mm) is incorrect, as this change in leg length to offset could
only be achieved with a neck angle of 180°.
Likewise, answer A is incorrect (0mm, 4.0mm),
as this change in leg length to offset could only be
achieved with a neck angle of 90°. Answer D (3.3,
2.3mm) is incorrect, as this leg length to offset
change would be the result of a neck angle of
145°. Answer C (2.8, 2.8mm) is incorrect, as an
equal increase in leg length and offset could only
be achieved if a neck angle of 135° (45° to the
horizontal) is used. The key to answering this
question correctly is not a thorough understanding of trigonometry but rather a knowledge that
if the neck angle is 135°, increasing neck length
will increase leg length and offset in an equal
ratio, and that if the neck length is less than
135°, increasing neck length will increase leg
length less relative to offset.
45. Answer B. Calcar pivot
In a seminal orthopaedic paper, Gruen described
both zones of failure of a cemented femoral total
hip replacement stem and five different modes of
failure. Zone 1 describes the proximal lateral
bone–cement–implant interface, zone 4 the tip
and zone 7 the proximal medial aspect of the
femur. With calcar pivot mechanism of failure,
the stem pivots on the calcar and toggles (windscreen wiper effect). Although Gruen’s original
paper described loosening of cemented femoral
stems, the calcar pivot mechanism is more
common with uncemented stems with a large
collar like the uncemented Austin Moore
prosthesis.
Gruen TA, McNeice GM, Amstutz HC.
‘Modes of failure’ of cemented stem-type femoral
components: a radiographic analysis of
loosening. Clin Orthop Rel Res. 1979;141:17–27.
46. Answer E. Pistoning: Stem within cement
A distal cement fracture and a radiolucent line
between the stem and cement mantle in Gruen
zones 1 and 2 are classical of pistoning of the
stem within the cement (mode Ia). Mode Ib
pistoning: stem within bone results in radiolucency in all Gruen zones.
Gruen TA, McNeice GM, Amstutz HC.
‘Modes of failure’ of cemented stem-type femoral components: a radiographic analysis of
loosening. Clin Orthop Rel Res. 1979;141:17–27.
47. Answer D. Poor distal cement fixation
Cemented femoral stem fracture is a rare complication. Poor proximal stem fixation or a low neck
cut reduces the support for the stem proximally. If
the stem remains well fixed distally, the increased
moment arm due to loss of proximal medial support can result in catastrophic stem fatigue failure.
This was described by Gruen as mode IV cantilever bending with lucency around the proximal
zones (1, 2, 6 and 7). Increased body mass, elongated femoral heads and smaller stem sizes are all
risk factors associated with stem fracture.
61
James Gill and Majeed Shakokani
Gruen TA, McNeice GM, Amstutz HC.
‘Modes of failure’ of cemented stem-type femoral components: a radiographic analysis of
loosening. Clin Orthop Rel Res. 1979;141:17–27.
48. Answer A. Direct anterior approach
The direct anterior approach (DAA) has been
shown to have a lower dislocation rate following
total hip arthroplasty. Elevate d BMI increases the
risk of dislocation. Neurological disorders
including Parkinson’s disease increase the risk
of dislocation. There is an increased risk of dislocation following total hip arthroplasty for hip
fracture. Proposed theories for this include predisposition to falls, capsular laxity compared
with osteoarthritis and the technical challenge
of restoring leg length and offset in the context
of hip fracture. A history of lumbar spine fusion
has been shown to increase the risk of hip
dislocation.
Kunutsor SK et al. Risk factors for disloca-
tion after primary total hip replacement: metaanalysis of 125 studies involving approximately
five million hip replacements. Lancet Rheum.
2019;1:e111–e121.
Buckland AJ et al. Dislocation of a primary
total hip arthroplasty is more common in
patients with a lumbar spinal fusion. Bone Joint
J. 2017;99-B:585–591.
49. Answer B. Removal of all metalwork and
cemented total hip arthroplasty bypassing the
most distal screw hole by at least 2.5 femoral
diameters
The cemented stem must pass well below the
distal screw hole to reduce the risk of periprosthetic fracture due to a stress riser. In a biomechanical study, Panjabi et al. (1985) showed that
normal stress patterns return at a distance of two
times the cylinder (femur) diameter away from
the stress riser.
Panjabi MM, Trumble T, Hult JE,
Southwick WO. Effect of femoral stem length
on stress raisers associated with revision hip
arthroplasty. J Orthop Res. 1985;3:447–455.
50. Answer A. Increased fracture toughne ss
Highly cross-linked polyethylene (HXLPE) has a
lower fracture toughness, decreased tensile
strength and decreased fatigue strength
compared with standard cross-linked polyethylene. HXLPE has the theoretical advantage when
used in total hip arthroplasty of increased wear
resistance. In total knee arthroplasty, fracture
and fatigue resistance are also important, and
so the benefits of HXPLE are not thought to
apply to total knee arthroplasty.
51. Answer B. 4cm
Leg lengthening of more than 4cm is generally
accepted to significantly increase the risk of sciatic nerve injury. Edwards et al. (1987) were the
first to report the increased risk of sciatic nerve
injury associated with leg lengthening of more
than 4cm. In a case series of 23 sciatic and
peroneal nerve injury palsies, they showed sciatic
nerve palsy occurred following mean lengthening
of 4.4cm (range 4 –5.1cm, n=3) and peroneal
palsy occurred with a mean lengthening of
2.7cm (range 1.9 – 3.7cm, n=12).
Edwards BN, Tullos HS, Noble PC.
Contributory factors and etiology of sciatic nerve
palsy in total hip arthroplasty. Clin Orthop Rel
Res. 1987;218:136–141.
52. Answer D. Plain film imaging
If the sciatic nerve has not been identified during
the operation, then surgical exploration is mandated, as it could have been injured or accidentally
tethered with a suture. Plain film imaging of the
hip is sensible before surgical exploration to assess
for leg lengthening, retained cement in the vicinity
of the nerve or a prominent acetabular screw in
the posterior inferior quadrant of the acetabulum.
If the sciatic nerve was identified, protected
throughout surgery and checked before closing
the fascia lata, then the surgeon may wish to
perform three-dimensional imaging to rule out a
haematoma or cement compressing the sciatic
nerve. If imaging is negative for occlusive pathology, one may wish still to explore the sciatic
nerve or observe the palsy for improvement.
53. Answer E. Larger femoral head
The primary arc range is the range of motion
which components allow between the two
extremes of impingement before the head begins
to lever out of the cup. The range of motion
allowed before the hip dislocates is termed the
lever range. The jump distance or excursion
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Hip II Structured SBA
distance is the distance the femoral head must
translate out of the cup in order to dislocate. In a
hemispherical cup, the excursion distance is
approximately equal to the radius of the femoral
head. Impingement occurs between the acetabular cup and the femoral neck and is dependent
upon cup geometry, head size and neck size. The
larger the head size is compared with the neck
(head–neck ratio), the greater the primary arc
range and stability will be. Increased offset and
increased femoral neck length have no effect on
the primary arc range. Extended lipped liners
and constrained liners reduce primary arc range.
When performing total hip arthroplasty, one
should aim to centre the primary arc range
within the patient’s functional hip range.
54. Answer B. Adhesive wear
Adhesive bearing wear is the most important process that generates submicron-sized polyethylene
(PE) particles. Adhesive wear occurs when the
anatomical forces between two opposing surfaces
are stronger than the inherent strength of either
material. In a metal-on-PE THA bearing surface,
adhesive wear results in small portions of the PE
surface adhering and transferring to the opposing
metal femoral head. This leads to wear particle
generation and the creation of pits and voids in
the PE. Abrasive wear occurs when a soft material
comes into contact with a hard material and the
microscopic counter-face asperities of the harder
material surface plough into the soft surface.
Corrosive wear is defined as the unwanted dissolution of a metal in a solution. Four modes of wear
have been described (Table 4.1). Inmode 1, thetwo
bearing surfaces are in contact with each other in
the manner intended bythe designer. In modes 2, 3
and 4, unintended surfaces are incontact with each
other, representing malfunctioning of the prosthesis. In mode 2 wear, a bearing surface is wearing
against a non-bearing surface, e.g. when the femoral head wears completely through the PE liner
and contacts the metal shell or cement. In mode 3
wear, the primary bearing surfaces are still articulating with each other but with the addition of an
interposed third body, e.g. cement, bone, metal or
ceramic fragments. In mode 4 wear, two nonbearing surfaces are moving against each other,
e.g. femoral neck and socket or ‘backside’ wear
between an acetabular liner and shell or fretting
Table 4.1 Mathematical analogues of the four wear modes
Surfaces in contact Mathematical
analogue
Primary vs. primary 1 1=1 1
Primary vs. secondary 1 2=2 2
Primary vs. primary vs.
third bodies
Secondary vs.
secondary
1 1 3=3 3
2 2=4 4
Mode
number
between the metal stem and the surrounding bone
or cement mantle.
In Table 4.1, a primary bearing surface is
assigned the number l; a secondary, non-bearing
surface is assigned a 2, and third body particles
are assigned a 3. Multiplication is the mathematical analogue for two or more surfaces in moving
contact to cause wear.
McKellop HA.The lexicon of polyethylenewear
in artificial joints. Biomaterials 2007;28:5049–5057.
55. Answer C. Insert screws into cup
The incidence of periprosthetic acetabular fractures recognised intraoperatively is approximately
0.4%. Occult fractures occur more frequently. A
study which reviewed CT scans performed as a
part of a study to review component position
reported an 8.4% incidence of periprosthetic acetabular fractures (Hasegawa et al. 2017).
Most periprosthetic acetabular fractures
occur during cup insertion and involve the
superior wall. Periprosthetic acetabular fractures
have been classified by Paprosky (Table 4.2). If
the fracture is undisplaced and the cup is stable
no further action is required. In reality, assessing
cup stability intraoperatively can be challenging
so use of supplemental screws through an uncemented cup is advised (Chitre et al. 2013); if the
cup is unstable and the fracture is displaced then
the fracture needs to be stabilised. Once the
fracture has been stabilised an assessment must
be made as to whether the bone stock is sufficient
to allow a component to be implanted. If there is
insufficient bone, bone grafti ng or the use of
trabecular metal augments should be considered.
The best method of treating a periprosthetic
fracture is to prevent it from occurring. This
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