1 General principles(a): The two wings of the pelvis are joined to the sacrum behind (1) by the very strong sacroiliac ligaments. In front they are united by the symphysis pubis (2). This arrangement forms a cylinder of bone, the pelvic ring (3), which protects the pelvic organs and transfers the body weight from the spine to the limbs. (The dotted line indicates the path of weight transfer.)

2 General principles (b): If the pelvic ring is broken at two levels, the pelvis may be free to open out or displace proximally (1), carrying the limb with it. Isolated (single) injuries of the ring (2) do not have this tendency. This simple concept underlines the main consideration in classifying pelvic fractures, namely to decide whether the injury is stable or unstable. Conversely, if there is a major pelvic displacement, the pelvic ring must be broken at two levels even if on first inspection only one is obvious.

3 Anatomical considerations (a): If the pelvis is subjected to splaying forces, the weak symphysis tends to give first (1); with greater violence, the sacrospinous (2), the sacrotuberous (3), and the anterior sacroiliac ligaments (4) rupture, so that the halves of the pelvis can externally rotate. If the pelvis is subjected to vertical shear forces, the interosseous (5) and posterior sacroiliac ligaments (6) are torn, leading to dislocation of the sacroiliac joint and proximal migration of the hemipelvis. In many injuries local fractures replace ligament tears (7).

4 Anatomical considerations (b): The urogenital diaphragm (1) is pierced by the membranous urethra (2), and the bladder (3) lies behind the pubic bones. Fractures of the pelvis, especially in the region of the symphysis and pubic rami, are often associated with damage to the urethra or bladder: rarely there may be involvement of the rectum.

5 Anatomical considerations (c): The pelvis is richly supplied with blood vessels which are frequently damaged by fractures. Internal haemorrhage is often severe, leading to shock which can be rapidly fatal. NB: Shock should be anticipated in pelvic fractures and merits first consideration (see p. 32). In the region of the sacroiliac joint the lumbo-sacral trunk and the sacral nerves are also susceptible to injury.

8 A1: Fractures not involving the pelvic ring: Avulsion fractures (a): (1): Sudden muscle contraction, especially in athletes, may avulse the anterior superior spine (sartorius) (1), the anterior inferior spine (rectus femoris) (2), the ischial tuberosity (hamstrings) (3), the posterior spine (erector spinae) (4), and the crest (abdominal muscles) (5). The crest and blade (6) may also be fractured by direct violence. In all cases there is localised tenderness, and symptomatic treatment is all that is usually required.

9 A1: Fractures not involving the pelvic ring: Avulsion fractures (b):

In the case of the ischial tuberosity, non-union is an appreciable risk, and if this occurs there may be problems with chronic pain and disability. To prevent this, open reduction and internal fixation have been advocated, and this is especially indicated if there is much separation of the fragment (2 cm or more).

10 A2: Minimally displaced, stable fractures of the pelvic ring (a): These like many other pelvic fractures may result from a fall on the side (1), force being transmitted through the trochanter or iliac crest, or from crushing accidents (e.g. from reversing vehicles (2), rock falls, etc.), or through violence transmitted along the femoral shaft (e.g. road accidents, falls from a height, etc.). Other stable injuries may follow anterior compression forces.

11 A2: Minimally displaced, stable fractures of the pelvic ring (b): By far the commonest fracture is one involving the superior pubic ramus. This is seen frequently in the elderly patient following a fall on the side. Osteoporosis or osteomalacia are frequently contributory factors. This injury is often missed and should always be suspected if there is difficulty in walking after a fall, and a femoral neck fracture has been excluded. Clinically there will be tenderness over the superior ramus, and pain on side-to-side compression of the pelvis; movements of the hips may be relatively pain-free. Radiographs of the hip may fail to visualise the area and full pelvic films are required. Treatment is by non weight bearing for 2–3 weeks until the pain settles and mobilisation can be commenced.

12 A2: Minimally displaced, stable fractures of the pelvic ring (c): Other fractures of this type include: (1) fractures of two rami on one side; (2) fracture of the ilium running into the sciatic notch; (3) fracture of the ilium or sacrum involving the sacroiliac joint. The nature of these injuries often suggests at least momentary involvement elsewhere in the ring: nevertheless, assuming that they are uncomplicated, they may be treated as in the previous frame.

13 A2: Minimally displaced, stable fracture of the pelvic ring (d): Many quadripartite (butterfly) fractures (1) of the pelvis, and double fractures of the rami with symphyseal involvement (2) are stable. They usually result from AP compression of the pelvis which is insufficient to disrupt the anterior sacroiliac ligaments: the posterior part of the pelvic ring remains intact and rigid, while the loose anterior fragment is only slightly displaced. Anticipate shock and damage to the urethra, but otherwise treat with 6 weeks’ non weight bearing.

14 B1: Anteroposterior compression, rotationally unstable, vertically stable fractures:(a): These injuries frequently result from run-over incidents where the wheel of a motor vehicle forces the anterior superior iliac spines backwards and outwards (1); they may also occur when the pelvis is compressed by falling rock or masonry which pins the patient to the ground. The ring fails anteriorly (e.g. at the symphysis or through two rami) (2), and posteriorly (3). The pelvis opens out (hence the term open-book fractures).

15 B1 (b): As the pelvis opens out the sacrospinous and sacrotuberous ligaments are torn, and CT scans may show anterior widening of the sacroiliac joints. The posterior sacroiliac ligaments remain intact, so that there is no vertical instability. Treatment:(a) Where the posterior damage is minimal and the anterior gap is less than 2.5 cm, treat by bed rest and monitor the symphyseal separation by repeated radiographs. (b) If the separation is greater or there is any significant associated sacroiliac separation, internal fixation is recommended.

17 Pelvic compression and stabilisation (a): A pelvic binder may be quickly and easily applied in the emergency room. It is slipped under the pelvis (which is then manually compressed) before fastening it in front. The London Splint (illustrated) uses a single Velcro fastener. Other binders include the Dallas Pelvic Binder and the Geneva Belt. (Note they are not suitable for prolonged use.) If any of these devices are applied before radiographs are taken they may mask the extent of the pathology.

18 Pelvic compression and stabilisation (b): An external fixator may be used. Fixation pins may be inserted (i) in the iliac crest, or (ii) in the anterior inferior iliac spine. (i) The iliac crest is exposed 2.5 cm posterior to the ASS through a 1.5 cm incision directed towards the umbilicus (1). Separate the subcutaneous fat down to bone with forceps, and identify the inner and outer tables. As the space between them is small, their orientation must be clearly defined. To do this, carefully introduce a 1.6 mm Kirschner wire on each side to act as a guide, keeping its tip against the bone (2). The crest is then drilled between them, using a guide (3) to protect the soft tissues. The fixation pins may be self drilling threaded Schanz pins of 5–6 mm in diameter, inclined at 45° to the body axis (4). Preferably two further pins, 2 cm apart are introduced in a similar manner on each side. They are connected to bars parallel to the crest (5). Before tightening the connector (6), close the pelvic gap by manual compression or by turning the patient on his side. This technique is suited to the emergency room. If available an intensifier may be used to help avoid poor placement of the pins. (Note that use of a C-clamp (see later) is often easier to apply.)

19 Pelvic compression and stabilisation (c): (ii) Alternatively (and the authors’ preferred technique) a threaded Schanz pin is inserted in to the anterior inferior iliac spine on each side. The quality of the fixation is better (and as is aptly remarked, it is easier to close a book from the front than the top). Method:A 2 cm incision is made 3 cm distal and slightly medial to the ASS. The tissues are parted down to bone with artery forceps and separated with small retractors, taking care to avoid damage to the lateral femoral cutaneous nerve (1). Use a drill sleeve while starting to drill the cortex. The screw should lie between the bony cortices, and above the acetabular roof (2). Its position should be checked with an image intensifier (although some rely on Kirschner wire guides as previously described). Use a single 6 mm self drilling or similar threaded pin, inserted to a depth of 40–50 mm (and if preferred, a second, inserted 2 cm proximal to the first). Reduction may be carried out under intensifier control, e.g. with manipulation of the Schanz pins; rotation of the hips to correct any rotation of a hemipelvis; longitudinal traction to the limb where there is any proximal displacement; direct lateral compression to close the pelvis before final tightening of the connectors. A curved radiolucent carbon fibre connector may also be used.

21 Pelvic compression and stabilisation (d): If the patient is not haemodynamically unstable, immediate and definitive conservative treatment is also possible using a canvas pelvic sling. The upper edges of the sling are reinforced with steel rods and eyelets, and it may be padded with wool prior to being positioned under the pelvis. Weights in the order of 4–8 kg are applied through pulleys in a Balkan beam (1), resulting in an inward thrust on the sides of the pelvis. If check radiographs show the reduction is incomplete, the weights may be increased or the traction cords crossed (2). The treatment should be continued for 6 weeks, taking the usual precautions over skin care; weight bearing can generally be allowed after 8 weeks.

22 Internal fixation (a): This radiograph shows a Type B1 open book fracture of the pelvis with wide separation at the symphysis pubis. Indications for open reduction and internal fixation of the symphysis include: 1. A symphyseal gap of 3 cm or more; 2. An associated S-I joint disruption; 3. A posterior pelvic disruption which requires stabilisation. An indwelling catheter should be introduced prior to surgery, but note that the presence of a suprapubic catheter or a colostomy are relative contraindications for surgery in this region.

23 Treatment (b): Check radiographs of the previous case show complete reduction of the symphyseal disruption which has been held with a reconstruction plate. This has been applied to the superior aspects of the pubic bones with three screws on each side. In similar cases, if there is any instability in the vertical plane, a second anterior plate may be required, although anterior plating is generally best avoided as there is some risk of the screws penetrating the retro-pubic space. (Meticulous care must be exercised in performing surgery in this area to lessen the risks of infection and disabling symphyseal osteitis.)

24 Ipsilateral lateral compression injuries (B2): Violence to the side of the pelvis (1) may cause fracture of the rami (2) or overlapping of the pubic bones; the posterior sacroiliac ligaments remain intact (3), and the hemipelvis hinges round them, often with crushing of the anterior margin of the sacroiliac joint on the same side as the rami (4). Tissue elasticity often leads to spontaneous reduction, so that in most cases no fixation is needed. In the remaining cases, an external fixator may be used to control any instability. Nevertheless an external fixator will be required in any case where there is a problem with haemodynamic instability, and should also be considered in cases of multiple injury, or where there is a problem with pain on mobilisation. Open reduction is required in the rare tilt fractures with perineal bone protrusion or significant pelvic asymmetry.

25 Contralateral compression injuries (bucket handle fractures) (B3): Two rami fracture (1) opposite the posterior injury (2) (or there is a butterfly fracture). The hemipelvis rotates (3), causing leg shortening without vertical migration. The majority are stable so that in most no fixation is needed. As in the previously described B2 cases, an external fixator may be used to control any frank instability or considered as suitable for dealing with leg shortening in excess of 1.5 cm. An external fixator will be required in any case where there is haemodynamic instability, multiple injuries or difficulty in pain control. Later, internal fixation should be considered where there is symphyseal separation or marked pelvic asymmetry.

26 Rotationally and vertically unstable injuries (C1, 2 & 3)(a): The pelvic ring is completely disrupted at two levels or more. Anteriorly, in the unilateral (C1) injury the symphysis is disrupted (1) or the rami fractured (2). Posteriorly, there is total loss of continuity between the sacrum and pelvis. This may result from complete ligamentous rupture and dislocation of the sacroiliac joint (3); or it may involve fractures of the posterior iliac spines (4), the ilium at other levels, or the sacrum. In bilateral (C2) injuries (5), there is extensive disruption (often with an anterior butterfly fracture (6)) involving both sides of the pelvis, so that both are free to migrate proximally.

27 Rotationally and vertically unstable injuries (b): On the radiographs the most obvious sign of vertical instability is the proximal migration of a hemipelvis. In addition, there may be avulsion of the transverse process of L5, and avulsion of the tip of the ischial spine. In the CT scan, posterior displacement of a hemipelvis by more than 1 cm is diagnostic. Note in the above radiograph gross vertical instability on the left, with fracture of both rami and dislocation of the sacroiliac joint. There is, in addition, disruption of the symphysis pubis and a fracture through the sacrum on the right (most noticeable at the inferior margin of the sacroiliac joint). Although not clearly shown, the transverse processes of L5 are fractured on both sides. This is a C2 (bilateral) fracture.

28 Rotationally and vertically unstable injuries (c): Initial treatment (IP): These serious injuries are often associated with life-threatening internal haemorrhage and other complications. In the initial stages, prompt application of an external fixator and skeletal traction may be sufficient to control blood loss, but in some cases a C-clamp (see next frame) or other measures (see Frame 35) will be required. Prior to surgery, or if surgery is considered inadvisable, control rotational instability with an external fixator, pelvic binder or canvas sling, and proximal migration of the hemipelvis with skeletal traction (up to 20 kg) applied through a supracondylar or tibial Steinman pin.

29 The anti-shock pelvic C-clamp (a): This consists of side arms (1) which can be moved freely along a cross bar (2) when pressure is applied proximally (3), but lock when subjected to any distal force (4). Hollow bolts (5) which screw into the side arms are designed to carry Steinman pins. Method of application:make a skin incision on each side approximately 6 cm or so from the posterior superior iliac spine, on a line connecting the posterior superior iliac spine with the anterior superior iliac spine (6), and slide the sidearms until the threaded bolts enter the wounds. Anchor each bolt by hammering a Steinman pin a depth of 1 cm into bone (7). Bring the bolts into contact with the pelvis by approximating the side arms with pressure from above. Tightening the bolts with a spanner will now close the S-I joints (8).

30 The anti-shock pelvic C-clamp (b): This radiograph is of the pelvis of an unrestrained passenger in a roll-over motor vehicle accident. It shows a disruption of the right sacroiliac joint and symphysis pubis with wide lateral displacement of the hemipelvis. On the other side there is displaced transverse fracture of the acetabular floor, and a fracture of the femoral shaft. A C-clamp was applied at an early stage, but in spite of this and a massive fluid replacement, hypotension and tachycardia remained a problem. An urgent laparotomy was performed: no hepatic or mesenteric vessel lesions were found, but there was a large retroperitoneal haematoma. The pelvis was packed and an anterior external fixator applied.

31 The anti-shock pelvic C-clamp (c): This is a screening film of the previous case, taken in the embolisation suite where the patient was taken because of persisting haemodynamic instability. It shows how the C-clamp has successfully reduced the sacroiliac joint separation. It also demonstrates the catheter and dye in the internal iliac artery. Its embolisation was finally successful in stabilising the hypotension, and the femoral fracture was then dealt with by intramedullary nailing. (48 hours later the sacroiliac joint and the symphysis pubis were internally fixed. Reconstruction of the acetabulum on the left had to be delayed for 4 weeks until the patient had fully recovered from a number of post-operative complications which included liver and renal failure.)

32 The anti-shock pelvic C-clamp (d): This CT scan shows marked widening of the right sacroiliac joint in another patient. This was also associated with disruption of the symphysis pubis and haemodynamic instability. As before it was considered important to stabilise the pelvis as soon as possible both from circulatory considerations and from the point of view of facilitating nursing during the initial period following the injury. (This is especially important in the cases of multiple injury and damage control orthopaedics.)

33 The anti-shock pelvic C-clamp (e): In this case, application of a C-clamp successfully stabilised both the sacroiliac joints and the haemodynamic instability. (Note in the scan only the tips of the screws are showing in the plane of the cut.)

Definitive treatment (a): Any definitive treatment procedure is usually advised within 5 days; protracted use of external fixation is unreliable, carries the risk of pin track infections and ultimately makes nursing less easy. Binders are not suitable for long-term use. The interval can be used with profit to stabilise the patient, to allow appropriate radiographs to be taken, and to obtain and check any specialised equipment required for the intervention.

34 Definitive treatment (b):Illustrated:the definitive treatment of the case shown in the previous two frames, using a reconstruction plate for the symphysis and two screws to secure the sacroiliac joint. Note that surgery should only be undertaken by the experienced, because of the high operative morbidity: problems include uncontrollable haemorrhage, infection, wound break-down, and neurological damage. Anterior approaches, which seldom give problems with wound healing, may be used to internally fix most fractures and dislocations in the region of the sacroiliac joints, to apply reconstruction plates to the ilium, to plate the rami and symphysis, and to reconstruct certain acetabular fractures.

35 Definitive treatment (c):Local posterior approaches may be employed to treat dislocations of the sacroiliac joint, and fractures of the sacrum in Type C injuries. A tension plate may be used: this is inserted through two small incisions (1) and a tunnel in the soft tissues made from below one posterior superior iliac spine to the other. The plate can be pre-bent to fit (2). Three cancellous screws should then be placed in each ilium (3), directed away from the sacroiliac joints and the dangerous area of the sacral ala (the lateral sacral masses).(d): The sacroiliac joint may be stabilised by screws running into the lateral ala (4). The position and direction of these screws must be determined with the greatest care, and it is advisable to visualise the ala directly and to use an image intensifier. In some cases, however, it is possible to carry out the procedure percutaneously, using guide wires and cannulated screws (as in Frame 32). (e): Where the sacrum is fractured, fixing screws must engage the body of the sacrum (5) to obtain the necessary purchase. Again the utmost care must be taken to avoid broaching the sacral canal (6) or entering the pelvis anteriorly (7). Formal posterior approaches (e.g. Kochers’s) may be needed in acetabular reconstructions where there is involvement of the posterior column. Note: In Type C3 injuries the acetabulum takes priority.

36 Definitive treatment (f):Formal posterior approaches (e.g. Moore’s) may be needed in some acetabular reconstructions where there is involvement of the posterior column or sciatic nerve involvement (See p. 289.)

Type C3 Injuries: Definitive treatment (a):In Type C3 injuries the acetabulum takes priority in the assessment and in any proposed reconstruction. (See also Frame 43 et seq.) The radiograph shows a type C3 pelvic injury. There is a widely displaced pubic symphysis, a fracture of the left superior pubic ramus, and a disruption of the left sacroiliac joint. There is a transverse fracture of the right acetabulum, and a dislocation of the right hip. This injury was treated initially with an external fixator and a manipulative reduction of the hip.

37 Type C3 Injuries: The urinary tract required investigation and a retrograde urethrogram was performed. This showed an intact urethra and normal bladder filling without spillage. (Illus.)

Definitive treatment (b): In view of the severity of the displacement two plates were applied to the symphysis, with one extending across the pubic ramus fracture to the ilium. Two plates were also used to secure the left sacroiliac joint, and two to fix the posterior aspect of the acetabulum.

38 Fracture of the sacrum (a):The ala of the sacrum (1) may be fractured as a result of either side-to-side or anteroposterior compression of the pelvis. Transverse fractures of the sacrum usually result from falls from a height on to hard surfaces. These fractures may be undisplaced (2), or they may be associated with some lateral (3) or anterior displacement (4).

39 Fractures of the sacrum (b): In some cases of both horizontal (A) and vertical (B) fractures there may be involvement of sacral nerve roots, leading to sensory disturbance, weakness in the leg(s) or saddle anaesthesia and incontinence. Treatment of the sacral fractures is symptomatic, with 2–3 weeks’ bed rest. In the majority of cases the neurological disturbance is transitory (generally being a lesion in continuity), but some advocate early investigation by CT scan, and dependent on the results, a laminectomy.

40 Coccygeal injuries (a): The coccyx is usually injured by a fall in a seated position against a hard surface, e.g. the edge of a step. There is local pain and tenderness, and sometimes pain on defaecation. Sitting on hard surfaces is always painful. (Note that coccygeal pain may also be caused by lumbar disc prolapse, pelvic inflammatory disease, chordoma and other tumours of the sacrum.)

41 Coccygeal injuries (b): The main patterns of injury are: (1) fracture of the coccyx, with a varying degree of anteversion, (2) anterior subluxation of the coccyx, (3) anterior displacement of the coccyx secondary to fracture of the end piece of the sacrum, (4) rarely, posterior subluxation of the coccyx.

43 Fracture of the acetabulum/central dislocation of the hip: In this group of injuries the pelvis is fractured as a result of force transmitted through the femoral head. This may occur from a blow or fall on the side (1) or when force is transmitted up the limb (2) via the foot, as in a fall from a height or through the knee as in a car dashboard injury.

44 Diagnosis (a): The fracture is easily diagnosed by routine radiographs which reveal characteristic disturbance of the acetabulum. There are, however, many patterns of injury due to variations in the amount and direction of the causal force. Pending full assessment, the patient should be treated for any accompanying shock, and skin traction (c. 4 kg) applied to the limb.

46 Is the anterior column intact? (a): The anterior column is the mass of bone which stretches downwards from the anterior inferior iliac spine, and includes the pubis and the anterior part of the acetabular floor. To help clarify this, CT scans are invaluable, but if this facility is not immediately available, then a three-quarter internal oblique view of the hip should be obtained.

47 Is the anterior column intact? (b): Foam wedges are placed under the buttock on the affected side until the pelvis is tilted at 45° (1). The rotation of the pelvis from the AP plane (3) to the oblique position (4) places the blade of the ilium in profile (5) to the central ray (2). The radiographic appearances are complex, especially if the fracture is badly displaced, but note on the films the state of the shaded portion (6) which represents the anterior column. Note also the easily identified posterior margin of the acetabulum (7), its less obvious anterior margin (8) and its floor (9). (10 and 11 are the anterior superior iliac spine and the iliac crest.) Isolated fractures of the anterior column are uncommon, but are important to recognise as they require an anterior surgical approach for reduction.

48 Is the posterior column intact? (a): The posterior column stretches upwards from the ischial tuberosity to the sciatic notch, and includes the posterior part of the acetabular floor. To clarify this a second oblique projection is often helpful (three-quarter external oblique projection).

49 Is the posterior column intact? (b): The pelvis is tilted 45° towards the injured side by sandbags under the good side (1). This rotation of the pelvis from the normal position (2) to the oblique (3) throws the area of the posterior column (4) away from the rest of the pelvis. Note on the radiographs the area of the posterior column (shaded), the ischial spine (5), and the anterior lip of the acetabulum (6). Careful study of the AP, lateral and two 45° obliques may allow you to assess the state of the columns. If CT scans are not available, stereoradiography or intensifier screening may be of further help.

Where they are, there is less difficulty in interpreting the exact nature of any fracture in this area, and 3-D reconstructions are of particular value. These will usually reveal the main elements involved and any displacement of the femoral head with great clarity; they should be used without hesitation.

50 What damage has the acetabular floor sustained? (a): The radiograph shows gross comminution of the acetabulum. The degree of acetabular disruption, and the extent of comminution may be further elucidated by CT scans or 3-D reconstructions.

51 (b): This CT scan shows an intact hip on the right side (see markers). On the left there is disruption of the medial part of the floor of the acetabulum, with an appreciable gap between the main fragments. There is comminution of the floor, with a bone fragment – equivalent to a loose body – lying between the femoral head and the main anterior fragment. There is a fracture of the anterior acetabular margin; this is not seen to be significantly displaced at this level of cut.

52 (c): A 3-D reconstruction of the previous case confirms the degree of comminution of the acetabular floor. This also shows with great clarity a backward projecting spike of bone in the region of the greater sciatic foramen. (This was held responsible for an accompanying sciatic nerve palsy, and was removed at open operation, during which internal fixation was carried out.) You may now be in a position to relate your findings to the AO Classification of acetabular fractures.

53 AO Classification of acetabular fractures: Type A: In Type A fractures one column only is involved, the other remaining intact.

A1: fractures of the posterior wall and certain variations; these injuries may be associated with posterior dislocation of the hip (see p. 295).

A2: fractures of the posterior column and variations.

A3: fractures of the anterior column and variations.

54 AO Classification of acetabular fractures: Type B: In Type B fractures the main fracture line lies transversely. Part of the acetabular roof always remains in continuity with the ilium.

B1: transverse fracture with or without involvement of the posterior acetabular wall.

B2: a vertical fracture added to the transverse fracture forms a T-fracture, of which there are several variations.

B3: a vertical fracture added to the transverse fracture involves the anterior column.

55 AO Classification of acetabular fractures: Type C: In Type C fractures both columns are involved, and no part of the acetabulum remains in continuity with the ilium.

C1: the portion of the fracture involving the anterior column extends to the iliac crest.

C2: the portion of the fracture involving the anterior column extends to the anterior border of the ilium.

C3: the fracture involves the sacro-iliac joint.

56 Treatment (a):Is the displacement of the fracture minimal (i.e. less than 2 mm) or is the acetabular floor highly fragmented?:If so, treat the injury conservatively with traction (e.g. Hamilton-Russell of 3 kg) for 6 weeks, with active hip movements during this period; weight bearing may be permitted after a further 3 weeks. Some loss of hip movements is inevitable, but overall function is often very good. Further treatment will only be required if secondary osteoarthritis supervenes.

57 Treatment (b):Is there a fracture of one or both columns, with displacement in excess of 2 mm? Is there a fracture of the acetabular floor without gross comminution? Is open reduction and internal fixation technically feasible, and is it likely to lead to reasonable restoration of the acetabulum? – if so, then surgery (which has an 85% success rate) is indicated. (The illustration is of a case where a badly displaced acetabular fracture involving the posterior column has been satisfactorily treated with reconstruction plates.)