Lag Screw and Plate Fixation

Overview

Screws and plates are the basic implants of open reduction and internal fixation. The aim in an extra-articular fracture is to restore length, axial alignment and rotation, with enough stability for biologic healing and early motion of adjacent joints; in an intra-articular fracture the aim is anatomic reduction of the joint surface, interfragmentary compression and absolute stability (Rockwood).

• Absolute stability (anatomic reduction + interfragmentary compression) gives no motion at the fracture and leads to primary (direct) bone healing. Typical uses: articular fractures and simple (transverse, short oblique, spiral) diaphyseal fractures, especially of the forearm (AO, Rockwood).

• Relative stability (bridging, splinting) restores alignment without reducing each fragment and leads to secondary healing with callus. Typical uses: multifragmentary metaphyseal and diaphyseal fractures (AO, Rockwood).

• Mixing techniques, that is aiming for relative stability without anatomic reduction in a simple fracture pattern, increases the risk of failure (Rockwood).

Screw Mechanics (AO, Rockwood)

A screw converts rotation into linear motion and torque into compression between two objects (Rockwood).

• Cortical screws have a small pitch and shallow threads; cancellous screws have a wide pitch and deep threads and are used in metaphyseal bone with thin cortex (Rockwood).

• A 4.5-mm cortical screw can generate roughly 2000-3000 N of compression (AO).

• Cannulated screws are inserted over a guidewire (femoral neck, sacroiliac screws); they are weaker, costlier and have smaller thread depth than solid screws of the same outer diameter (Rockwood).

• Locking head screws have a threaded head that locks into the plate hole and create a fixed-angle construct; they have a larger core and shallower thread and cannot be used as lag screws (AO, Rockwood).

Functions of screws (AO)

Lag Screw Principle

A lag screw grips only the far cortex; the thread does not engage the near cortex. As the head bears on the near cortex, the far fragment is pulled toward it and interfragmentary compression results (Rockwood).

• Two ways: a fully threaded screw inserted with the lag technique (glide hole), or a partially threaded (lag-design) screw (Rockwood, Campbell).

• A fully threaded screw used as a lag screw allows the near hole depth to be adjusted and more thread engagement in the far cortex; this is important when lagging from a large near fragment into a small far fragment, where a partially threaded screw may leave threads across the fracture and give no compression (Rockwood).

• With partially threaded cancellous screws, all threads must lie beyond the fracture line in the far fragment; a washer may be required in thin metaphyseal cortex (femoral neck, distal femur, tibial plateau, medial malleolus) to prevent the head from sinking (Rockwood, Campbell).

• A single lag screw does not control rotation; at least two lag screws, or a lag screw protected by a plate, are needed (AO).

• The screw should be directed orthogonal to the fracture plane; otherwise screw torque creates shear at the fracture and displaces the reduction (Campbell).

• Interfragmentary screw fixation alone is well suited to avulsion fractures and to shear-type epiphyseal and metaphyseal intra-articular fractures (Campbell).

Technique 58.4: Inserting a 4.5-mm Cortical Lag Screw (Campbell)

1. Reduce the fracture and secure the reduction with forceps or provisional Kirschner wires.

2. Plan the screw in the middle of the fragment, equidistant from the fracture edges and orthogonal to the fracture plane.

3. Drill the near cortex with a 4.5-mm drill (glide hole).

4. Insert the drill reduction sleeve for the 3.2-mm drill and drill the far cortex with a 3.2-mm drill.

5. Countersink the 4.5-mm hole to give maximal contact with the screw head and disperse load on the cortex.

6. Measure the length with a depth gauge and cut threads in the far cortex with the 4.5-mm tap, so the threads hold only the far cortex.

7. Insert a screw of proper length and watch interfragmentary compression develop as it is tightened. Do not remove provisional fixation or the holding forceps until the screw is fully seated.

Position screw (Campbell): if a 4.5-mm cortical screw is to hold position without compression, drill both cortices with 3.2 mm, measure, tap 4.5 mm along the whole hole and insert the screw.

Technique 58.5: Cancellous Lag Screw (Campbell)

Insertion is similar to cortical screws except that the near cortex is not overdrilled; the unthreaded shank does not hold the near cortex.

1. In soft cancellous bone, insert a washer under the head to increase surface area and prevent the head from pulling through the cortex.

2. In firm, hard metaphyseal bone, tap as for a cortical screw but only in the near cortex.

3. 4.0-mm cancellous screw: 2.5-mm drill, 3.5-mm tap. 6.5-mm cancellous screw: 3.2-mm drill, 6.5-mm tap.

4. In osteoporotic, soft bone tapping may not be necessary.

5. Select a thread length that places all threads in the far fragment and none in the near fragment, otherwise compression is not achieved.

Plate Functions

The function of a plate is defined by how it is applied, not by its design; most plates can be used in several modes (AO, Rockwood).

Locking itself is not a plate function: a locking plate can be applied in neutralization, compression, buttress or bridge mode (Rockwood).

Four ways to obtain compression with a plate (AO)

1. Eccentric screw placement in a dynamic compression hole (load position).

2. Prebending (overbending) the plate, so the far cortex is also compressed.

3. A lag screw through the plate across the fracture.

4. An articulated tension device, used when the gap is more than about 2 mm or for nonunions.

Dynamic compression unit (AO)

• In the LC-DCP, eccentric (load) drilling moves the screw about 1 mm, giving compression as the head slides down the inclined hole.

• The gold (eccentric) drill guide gives compression; the green (neutral) guide places the screw in a neutral position.

• Screws can be angled up to about 25° longitudinally and 7° transversely.

Campbell's Concepts: Principles of Plate Fixation (Campbell)

• Plates neutralize deforming forces that cannot be counteracted by screws alone.

• Plates may require contouring to maintain optimal stability of the reduction.

• Screw placement and sequence are critical; incorrect placement or order causes displacement or shear and loss of reduction.

• Adequate screw purchase is required: usually six to eight cortices on both sides of the fracture, except with buttress plates.

• Plates should be long enough; the larger the bone and the greater the stresses, the longer the plate.

• Avoid overtorquing screws during insertion.

• Retighten all screws before wound closure to allow for stress relaxation at the screw-bone interface.

• With a prebent plate, insert the inner screws (near the fracture) first and then the outer screws; if outer screws are placed first, the near cortex opens.

• Tension band plating must be applied to the tension (convex) side; a plate on the compression (concave) side is likely to bend, fatigue and fail.

• In oblique fractures, fix the plate first to the fragment that forms an axilla with the plate, so that compression drives the opposite fragment into it; fixing the wrong fragment first produces shear (Rockwood).

Bridge Plating and Locking Constructs (AO, Rockwood)

• Bridge plating restores length, alignment and rotation without exposing the fracture; the plate is fixed only to the proximal and distal main fragments.

• Plate length: about three times the length of the fracture zone in comminuted fractures and eight to ten times in simple fractures (AO).

• Screw density (screws/holes) below about 0.5; leave two to three holes empty over the fracture to give a longer working length and spread strain (AO).

• Working length is the distance between the closest screws on each side of the fracture; a longer working length lowers construct stiffness (Rockwood).

• Locking screws are useful in osteoporotic bone, short metaphyseal segments and when the plate is not perfectly contoured; a nonlocking screw would pull bone to the plate and lose the reduction (Rockwood).

• Use at least two, preferably three, locking screws per main fragment; bicortical locking screws give much greater torsional resistance than unicortical ones in osteoporotic bone (Rockwood).

• Reduce and lag first, then lock: in hybrid constructs, insert nonlocking (compression, buttress) screws before locking screws (AO, Rockwood).

• Titanium plates are about 50% less stiff than stainless steel plates; bending stiffness increases with the cube of plate thickness (Rockwood).

• Stress risers occur at the end of a plate and at empty holes; ending a construct with a unicortical or angled nonlocking screw makes the transition more gradual (Rockwood).

Pitfalls

• Lag screw threads crossing the fracture: no compression.

• Locking head screw used where interfragmentary compression is needed: compression is not possible.

• Too few cortices or a short plate in diaphyseal fixation: early loosening.

• Overly rigid, short, densely screwed bridge construct over a simple fracture: delayed union or plate fatigue failure.

• Straight plate on a transverse fracture: gapping of the far cortex.

• Missing medial (opposite) cortical support in tension band plating: plate fatigue and breakage (Rockwood).

Implant Removal (Campbell)

These intervals apply to recent fractures with uncomplicated healing; nonunions, major fragments and post-infection cases are judged individually.

References

1. Whittle AP, Azar FM. General principles of fracture treatment. In: Azar FM, Sawyer JR, Throckmorton TW, eds. Campbell's Operative Orthopaedics. 15th ed. Philadelphia: Elsevier; 2026. Chapter 58, Techniques 58.4-58.5.

2. Screws; Plates. In: Buckley RE, Moran CG, Apivatthakakul T, eds. AO Principles of Fracture Management. 3rd ed. Stuttgart: Thieme; 2017. Chapters 3.2.1-3.2.2. (Turkish edition: AO Kırık Tedavisinin Prensipleri.)

3. Principles and biomechanics of internal fixation. In: Tornetta P III, Ricci WM, Ostrum RF, McKee MD, Ollivere BJ, de Ridder VA, eds. Rockwood and Green's Fractures in Adults. 10th ed. Philadelphia: Wolters Kluwer; 2024. Chapter 12. (Turkish edition: Erişkin Kırıkları.)