FRCS: Basic science

Vertebral fractures may give rise to false low density values. DEXA scanning is used to assess bone mineral density, in particular to diagnose osteoporosis. Vertebral fractures may give rise to falsely elevated bone density values.

III. There are several collagen types, including: Type I collagen is the predominant form in bone and fibrocartilage Type II collagen is found in articular cartilage Type III collagen is produced in the proliferative phase of tendon and ligament healing Type IV collagen forms the bases of cell basement membranes Type X collagen is found in mineralizing cartilage in endochondral ossification

Chondrocyte number increases in ageing. Chondrocyte numbers decrease in ageing cartilage. The others are all true: Osteoarthritis Ageing Water content Increases Decreases Proteoglycan degradation Increases Decreases Chondrocyte number Decreases Decreases Young’s modulus of elasticity Decreases Increases

Vibration is transmitted in the anterior corticospinal tract. Vibration is also transmitted in the dorsal columns. The anterior corticospinal tract is a motor pathway.

Transforming growth factors. Bone morphogenetic proteins (BMPs) are multifunctional growth factors that belong to the transforming growth factor beta (TGF-b) superfamily.

Parathyroid hormone (PTH) acts directly on osteoclasts to stimulate bone resorption via carbonic anhydrase. PTH is an 84-amino-acid peptide, produced by the chief cells of the parathyroid glands. Its overall effect is to increase serum calcium levels, by action in the kidney (stimulation of hydroxylation of 25(OH)-vitamin D3 and increased reabsorption of filtered calcium) and in bone (bone resoprtion). It acts indirectly on osteoclasts via a secondary messenger mechanism via osteoblasts. 236

Ca10 (PO4)6 (OH)2. Calcium hydroxyapatite (Ca10 (PO4)6 (OH)2) forms part of the inorganic component of the bone matrix and gives it compressive strength.

Adequate stride length. The five prerequisites of gait according to Gage are: Stability in stance Adequate step length Clearance in swing phase Foot prepositioning Conservation of energy Step length is the horizontal distance between foot contact to the next contralateral foot contact, as opposed to stride length which is the distance between foot contact to the next ipsilateral foot contact.

A bilateral above knee amputee (AKA) requires 80% increase in energy expenditure for ambulation. Unilateral BKA necessitates a 25–50% increase in energy expenditure. Bilateral AKA requires >200% increase.

It exhibits a high Young’s modulus. PMMA or bone cement is used widely in orthopaedics, principally with implants. It is strongest in compression, but its disadvantages include its poor tensile and sheer strengths. Its Young’s modulus is comparatively low (see question 18), and is between that of cortical and cancellous bone.

10. 1 point 2 points 3 points Site Upper limb Lower limb Pertrochanteric Size <1/3 diameter 1/3–2/3 >2/3 Symptoms Mild Moderate Functionally limiting Lesion type Blastic Mixed Lytic One way to help commit this scoring system to memory is just to remember the worst case scenario; a patient with a large, lytic, pertrochanteric lesion whose pain is preventing them from walking. In Mirels’ paper, a score of 9 gave a fracture risk of 33% (compared to 15% with a score of 8) and he therefore concluded that a score of 9 or greater should indicate the need for prophylactic fixation.

Eccentric contraction of gastro-soleus occurs during the second rocker. During the first rocker the ankle dorsiflexors are undergoing eccentric contraction to control the foot and prevent foot-slap after heel-strike. During the second rocker the body moves forwards with its momentum and the ankle plantarflexors undergo eccentric contraction to control mid stance. They then contract concentrically to provide power through push-off; this is the third rocker. 237

Cobalt chrome, stainless steel, titanium, cortical bone, PMMA cement, UHWMPE. Material Young’s modulus (approx. values in GPa) Ceramic 350 Cobalt chrome 210 Stainless steel 190 Titanium 100 Cortical bone 20 PMMA cement 2 UHWMPE 1 Cancellous bone 1 Tendon 0.5 Cartilage 0.02

The disease should be one in which late treatment is as effective as early treatment. The following criteria are accepted by the WHO (as described by Wilson and Jungner in 1968):
The condition sought should be an important health problem for the individual and community.
There should be an accepted treatment or useful intervention for patients with the disease.
The natural history of the disease should be adequately understood.
There should be a latent or early symptomatic stage.
There should be a suitable and acceptable screening test or examination.
Facilities for diagnosis and treatment should be available.
There should be an agreed policy on whom to treat as patients.
Treatment started at an early stage should be of more benefit than treatment started later.
The cost should be economically balanced in relation to possible expenditure on medical care as a whole.
Case-finding should be a continuing process and not a ‘once and for all’ project.

The zone of proliferating activity controls limb growth in a proximal to distal direction. The zone of proliferating activity influences limb growth in an anteroposterior direction which, as the limb has not yet rotated, equates to a radial-ulnar direction in the case of the upper limb.

Radiolucent lines in Gruen zones 1, 2, 6 and 7. Gruen described seven zones around a cemented femoral stem starting with zone 1 at the greater trochanter round to zone 7 at the calcar, zone 4 being at the tip of the prosthesis. He described different modes of failure of cemented stems: 238 Mode Mechanism Cause Findings 1A Pistoning Stem subsiding within cement Radiolucent lines in zones 1 and 2 1B Pistoning Stem and cement subsiding within bone Radiolucent lines in all 7 zones 2 Medial stem pivot Lack of supermedial and inferolateral support Medial migration proximally and lateral migration distally 3 Calcar pivot Medial-lateral toggling of distal stem Radiolucent lines in zones 4 and 5 4 Cantilever bending Loss of proximal support with a well-fixed distal stem Stem fracture, radiolucent lines on zones 1, 2, 6 and 7

Ewing’s sarcoma. Chondrosarcomas typically occur in middle age and have a predominance for the pelvis and shoulder. Osteosarcomas have a bimodal age distribution with a peak in childhood and the elderly. They most commonly occur in the distal femur and proximal tibia. Giant cell tumours generally occur after skeletal maturity. It usually appears as an eccentric, lytic, expanding lesion in the distal metaphysic/epiphysis. It is locally aggressive but rarely malignant.

The ligament of Struthers. The leash of Henry refers to the recurrent branches of the radial artery in the forearm. The arcade of Frohse is the tendinous proximal border of supinator. The tendinous origin of ECRB is a potential site of compression. The ligament of Struthers is a fibrous band extending from a large bony projection of the humerus, known as the supracondylar process, to the medial epicondyle. It is probably present in less than 1% of humans, and may cause median nerve compression.

The thoracodorsal nerve. The lateral pectoral nerve receives contributions from C5/6/7. The axillary nerve: C5/6. The upper subscapular nerve: C5/6. The thoracodorsal nerve: C7/8 and variably C6. The radial nerve: C5/6/7/8 and variably T1.

Isotonic. Isometric muscle contraction occurs when muscle contraction generates tension without a change in its length. Plyometric exercises are defined as a muscle stretch followed by a rapid contraction and is a very efficient method of exercising to improve power delivery. Isokinetic exercises are resistance-based exercises designed to provide a specific level of resistance while maintaining a consistent speed of limb movement. They require use of special equipment such as the Cybex machine. Closed chain exercises are where the distal portion of the involved limb is stabilized – this minimizes shear forces across the joint. They are commonly used in anterior cruciate ligament (ACL) reconstruction rehabilitation.

Z disc. The I band contains actin only. The A band contains actin and myosin. The M line is the interconnecting site of the thick myosin filaments. The H band contains myosin only. The Z disc anchors the thin actin filaments and represents the boundary between adjacent sarcomeres. 239

Clinical governance. Appraisal for consultants is designed to be a professional process of constructive dialogue in which the doctor being appraised has a formal structured meeting to reflect on their work and to consider how their clinical effectiveness might be improved. Revalidation is the process whereby the General Medical Council establishes a doctor’s fitness to practise and with it, the right to remain on the medical register. Licensing is the first step towards the introduction of revalidation. To practise medicine in the UK all doctors are required by law to be both registered and hold a license to practice. Clinical effectiveness is defined as the extent to which specific clinical interventions do what they are intended to do. It is described as the right person doing the right thing in the right way at the right time in the right place with the right result. Clinical governance is how health services are held accountable for the safety, quality and effectiveness of clinical care delivered to patients.

The graft’s three-dimensional scaffold for bone ingrowth. An osteogenic graft contains living cells capable of differentiating into osteoblasts etc. A graft which provides a biological stimulus for bone formation is said to be osteoinductive – e.g. if BMPs are included. Allograft and xenograft have immunogenic potential to sensitize patients. Osteoconductive refers to the graft’s three-dimensional scaffold for bone ingrowth.

A material that has an elastic limit that approximates its fracture point. A material that can undergo extensive plastic deformation is said to be ductile. Hardness is a surface property of a material. The amount of deflection for a given load relates to a material’s stiffness. The maximum amount of stress a material can withstand prior to failure is its ultimate tensile stress.

Changing to a finer pitch. The relationship of the inner to the outer diameter affects pull-out strength – a relatively smaller inner (or larger outer) diameter increases pull-out strength. A finer pitch allows for more threads to grip each cortex increasing pull-out strength. A locking screw/plate combination may increase the pull-out strength of the construct as a whole but the addition of the locking thread has no effect on the screw in isolation. 240