Posterolateral Anatomy in the Knee, Hip, and Spine

Posterolateral refers to the back and outer side of a body structure, and in medicine it shows up most often when talking about the knee. The posterolateral corner (PLC) of the knee is a dense web of ligaments, tendons, and capsular tissue that prevents the lower leg from twisting outward and bowing into a knock-kneed or bow-legged position. Injuries here have been found in roughly 16% of all knee injuries, yet they remain one of the most commonly missed diagnoses in orthopedics. The term also appears in spine and hip surgery, where “posterolateral” describes both a surgical approach and a pattern of disc herniation, each with distinct clinical significance.

What Lives in the Posterolateral Corner of the Knee

The posterolateral corner is not a single ligament. It is a layered arrangement of structures on the outside-back of the knee that work together to keep the joint from wobbling when you plant, pivot, or absorb force. Biomechanical studies have identified three primary stabilizers: the lateral collateral ligament (LCL), the popliteus tendon, and the popliteofibular ligament.¹ Several additional structures reinforce the area, including the posterolateral capsule itself, the arcuate ligament, and the fabellofibular ligament.²

Each of these structures has a slightly different job. The lateral collateral ligament is the main restraint against the knee opening outward (varus force). The popliteofibular complex is the most important structure for controlling outward rotation of the shinbone at all angles of knee bending, and it also helps resist varus force when the knee is partially flexed. The posterolateral capsule contributes to both varus and rotational stability when the knee is near full extension.³ Because these structures share duties across different knee positions, losing one can overload the others. That overlap is part of why isolated PLC damage is tricky to detect clinically and why partial injuries sometimes fly under the radar.

Why These Injuries Are So Often Missed

Isolated posterolateral corner injuries are rare and frequently go undiagnosed, often associated with sports trauma, car accidents, and falls.⁴ One reason for the diagnostic blind spot is that PLC damage rarely happens alone. It typically accompanies tears of the anterior cruciate ligament (ACL) or posterior cruciate ligament (PCL), and the attention understandably gravitates toward those better-known injuries. If the PLC damage is overlooked and only the cruciate ligament is repaired, the reconstruction is at higher risk of failing. Current literature identifies unrecognized PLC injury as a common cause of ACL reconstruction failure, and guidelines now recommend addressing high-grade PLC damage at the same time.⁵

Even in younger patients, PLC injuries co-occur with ACL tears. A study of skeletally immature patients (average age 14) who had ACL reconstruction found concomitant PLC injuries, though in that group there was no statistical association between PLC injury and ACL graft failure.⁶ The takeaway for any patient with a significant knee ligament injury is that the posterolateral corner should be specifically evaluated, not assumed intact.

How Posterolateral Corner Injuries Are Diagnosed

The main bedside test for PLC instability is the dial test, which measures how much the shinbone rotates outward compared to the uninjured leg. But the test has real limitations. Cadaveric research has shown that the dial test reliably detects instability when three or more posterolateral structures are torn, or when a cruciate ligament tear accompanies two posterolateral tears. However, when only one or two PLC structures are damaged, the dial test may not pick up the instability at all.⁷

When the dial test is positive, the degree of rotation correlates with the severity of damage. Clinical work grouping patients by rotational side-to-side differences (under 15 degrees, 15 to 20 degrees, and over 20 degrees) found that the number of positive physical exam and arthroscopic findings increased as the rotational difference grew.⁸ Tibial positioning during the test also matters: placing the tibia in a reduced (neutral) position versus a subluxed (shifted) position produces different measured angles, which can affect whether the examiner calls the test positive or negative.⁹

The Role of Imaging

MRI is the standard tool for evaluating the PLC, but the area’s complex and variable anatomy has earned it the nickname “the dark side of the knee.” Standard MRI is a static picture; it shows structural tears but does not reveal how much functional looseness those tears cause. Stress radiography, where force is applied to the knee during imaging, provides that dynamic piece. In patients with complete PLC injuries on MRI, varus stress radiographs showed an average opening of about 18.6 mm, compared to about 12.8 mm in partial injuries.¹⁰ That distinction between complete and partial injury helps surgeons decide who needs surgery and who might heal with bracing alone.

Newer research has explored varus stress MRI, which combines the structural detail of MRI with an applied load, allowing clinicians to evaluate both the anatomy and the functional laxity in a single exam.¹¹ For complex multiligament knee injuries, the consensus is that stress radiographs and MRI complement each other: MRI identifies which structures are torn, while stress imaging shows how much instability those tears actually create.

Surgical Reconstruction of the Posterolateral Corner

When a PLC injury is severe enough for surgery, the goal is to restore both varus stability and rotational control. Several reconstruction techniques exist, but they are not all equal. A cadaveric comparison of three methods found that “anatomic” reconstruction techniques, which rebuild the structures roughly where they naturally attach, were significantly more effective at controlling outward rotation than the older modified Larson technique.¹² Anatomic reconstruction also restored varus stability to levels that were not significantly different from the intact knee across multiple flexion angles.¹³

One question patients often have is whether the graft tissue matters. A meta-analysis comparing autograft (tissue harvested from the patient’s own body) to allograft (donor tissue) in PLC reconstruction found no difference in graft failure rates between the two types. However, autograft patients scored slightly higher on the Lysholm knee function scale after surgery (about 89.6 versus 85.5 for allograft), with no difference in other outcome measures like varus laxity or IKDC scores.¹⁴ The practical difference is modest enough that graft choice often comes down to the surgeon’s preference and whether the patient has usable donor sites.

The Common Peroneal Nerve Problem

One of the trickiest aspects of both PLC injuries and PLC surgery is the common peroneal nerve, which wraps around the top of the fibula right in the surgical field. This nerve controls the muscles that lift your foot and toes, and damage to it causes foot drop, a highly disabling problem. The nerve sits an average of about 12 mm from the posterolateral joint capsule, though that distance ranges from under 5 mm to over 22 mm depending on the individual.¹⁵ That wide range means the nerve is dangerously close to the surgical zone in some people and relatively safe in others, with no way to know in advance without careful intraoperative identification.

The risk increases in specific injury patterns. When the biceps femoris tendon avulses (tears off) from the fibular head, it can pull the common peroneal nerve forward with it into an abnormal position. A study of patients with PLC injuries found that 16 out of 18 patients who had biceps avulsions or avulsion fractures of the fibular head had a displaced nerve, while none of the 34 patients with proximal injuries (higher up) had abnormal nerve positioning.¹⁶ For surgeons, the message is clear: whenever a fibular head avulsion is suspected, the nerve is probably not where it should be.

Cadaveric work mapping the nerve’s position relative to bony landmarks found that the distance from the fibular head to where the nerve crosses the fibular neck averaged about 20.7 mm and stayed essentially constant whether the knee was straight or fully bent.¹⁷ That consistency gives surgeons a reliable landmark to avoid the nerve, at least in patients whose anatomy has not been displaced by injury.

Recovery After Posterolateral Corner Surgery

How aggressively a patient loads the knee after PLC reconstruction has been debated, and the evidence here is somewhat counterintuitive. A systematic review divided post-surgical protocols into three groups: no weight-bearing for the first four weeks, progressive weight-bearing over six weeks, and immediate weight-bearing. There was no significant difference in functional outcome scores among the three groups. But complication rates told a different story. The delayed weight-bearing group had the highest rate of arthrofibrosis (stiffness requiring manipulation under anesthesia) at 11%, along with an overall complication rate of 44%. The progressive weight-bearing group had the lowest complication rate at just 3% and arthrofibrosis rate of 0%. The immediate weight-bearing group fell in between.¹⁸

Return-to-sport timelines varied as well. Patients on progressive protocols returned at a mean of 6 months, compared to about 9 months for immediate and 10.5 months for delayed weight-bearing groups. The emerging picture favors a progressive approach: controlled, early loading appears to reduce stiffness and complications without sacrificing graft integrity or functional outcomes. Rehabilitation typically follows phased protocols that address pain first, then restore range of motion, rebuild strength, and finally train balance and proprioception.¹⁹

Posterolateral Corner Injuries in Children

PLC injuries in growing patients present a unique challenge because their bones are still developing. Unlike adults, where tears happen in mid-substance ligament, children tend to sustain avulsion injuries: the ligament pulls off a piece of bone from its attachment point rather than tearing through its fibers. In a series of six children with acute PLC injuries, five were treated with open reduction and fixation of the bony fragment, and functional scores were excellent (mean Lysholm score of 98 out of 100, with normal range of motion and knee laxity measurements).²⁰

The cautionary case in that series was a child treated nonoperatively who developed a bone bridge across the growth plate, leading to a valgus (inward-angled) deformity of the knee. That outcome underscores why surgical fixation of the avulsed fragment is generally preferred in pediatric patients: it restores the anatomy and reduces the risk of growth disturbance. Case reports of adolescents have also documented combined ACL and PLC injuries treated with acute PLC repair and delayed ACL reconstruction, allowing the growth plates to be managed more carefully.²¹

Posterolateral Pain Management During Knee Surgery

One of the practical challenges after any knee surgery involving the posterolateral region is pain control. Standard nerve blocks for knee surgery tend to target the front and inner portions of the joint, leaving the back-outer quadrant relatively under-covered. A recently described technique, the ultrasound-guided biceps femoris short head block, targets the nerve supply to the posterolateral knee capsule specifically. Preliminary data show complete blockage of the posterolateral capsule with this approach, and because it spares the motor nerves, patients retain the ability to move their foot and ankle normally after the block.²² This kind of targeted approach could become especially useful for total knee replacement patients, where pain from the posterolateral capsule is a recognized gap in many anesthesia protocols.

Posterolateral in the Spine

Outside the knee, “posterolateral” appears constantly in spine surgery and spinal pathology. Disc herniations, for example, most commonly occur in a posterolateral direction. The mechanics involve an initial failure of the inner annulus on the lateral side of the disc, followed by the nucleus tracking circumferentially before breaking through the back or back-outer wall of the disc.²³ Posterolateral herniations are common because the posterior longitudinal ligament, which runs down the center-back of the spinal column, reinforces the midline but thins out at the sides, creating a structural weak point.

In spinal fusion surgery, posterolateral fusion (PLF) is a long-established technique used to treat conditions like spondylolisthesis, where one vertebra slips forward on another. Bone graft is placed along the outer-back of the spine to encourage the vertebrae to grow together. Long-term follow-up of over ten years has shown that whether solid fusion is achieved does not strongly predict disability scores at that time point, and no revision surgeries for non-union were needed in one cohort.²⁴ However, other research looking specifically at degenerative spondylolisthesis found that achieving solid fusion did produce significantly better outcomes at five years and beyond, though not at the one- or three-year marks.²⁵ The practical takeaway is that fusion status matters for long-term results, even if patients feel similar in the first few years regardless.

Compared to transforaminal lumbar interbody fusion (TLIF), which places graft material inside the disc space, posterolateral fusion has a lower pooled fusion success rate: about 85% versus 94% for TLIF in a systematic review and meta-analysis.²⁶ PLF remains widely used because it is less technically demanding and avoids entry into the spinal canal, but for patients where achieving fusion is especially important, TLIF may offer better odds.

The Posterolateral Approach in Hip Replacement

In hip surgery, “posterolateral” most often refers to the surgical approach used to access the hip joint during total hip arthroplasty. The surgeon enters from the back-outer side of the hip, splitting muscles to reach the joint. The traditional concern with this approach has been a higher risk of posterior dislocation compared to approaches that come from the front or side. Newer refinements have addressed that. One technique, which preserves the piriformis tendon, the quadratus femoris muscle, and the gluteus medius while performing a direct capsular repair, combines a muscular and capsular “hammock” effect that limits posterior dislocation risk.²⁷

A prospective safety study compared patients managed with standard post-operative restrictions (no crossing legs, no bending past 90 degrees, elevated toilet seats) against patients given minimal precautions after posterolateral-approach hip replacement with femoral heads of 28 mm or larger. The less restricted group had zero dislocations; the more restricted group had one. The difference was not statistically significant, but the finding supports the idea that for experienced surgeons using modern implant sizing and soft-tissue repair, strict hip precautions may be unnecessary.²⁸ That is a meaningful quality-of-life detail for hip replacement patients, who traditionally spend weeks restricted from normal activities like sitting in a low chair or putting on socks.