ICD-10 Code S83.422A: Lateral collateral ligament sprain, left knee

Key takeaways

In short, ICD-10 Code S83.422A is the billable diagnosis code for a sprain of the lateral collateral ligament of the left knee, initial encounter.

The 7th character A confirms the patient is in active treatment for an acute injury. Otherwise, use D for follow-up visits and S for late effects.

Laterality and ligament are both required. Specifically, S83.429A drops the side and S83.402A drops the ligament, so neither substitutes for S83.422A.

Notably, category S83 carries no Excludes1 note, so every condition on its excludes list may be reported alongside S83.422A.

Finally, practice management software like Pabau helps orthopedic and sports medicine practices capture the codes a clinician documents and submit them with the claim.

ICD-10 Code S83.422A is the billable diagnosis code for a sprain of the lateral collateral ligament of the left knee, initial encounter. In practice, one seven-character string carries the ligament, the side, and the phase of care.

Notably, three siblings absorb the claims that lose any part of that detail. Specifically, S83.412A covers the medial collateral ligament, S83.429A drops the side, and S83.402A leaves the ligament unnamed. Overall, this reference covers code structure, encounter-type rules, sibling codes, excludes notes, CPT pairings, and documentation requirements.

ICD-10 Code S83.422A: definition and billable status

To begin, ICD-10 Code S83.422A is a valid, billable ICD-10-CM diagnosis code. Specifically, its official full description is Sprain of lateral collateral ligament of left knee, initial encounter. In addition, the Centers for Medicare and Medicaid Services, known as CMS, lists it as valid for FY 2026, effective October 1, 2025.

As a result, it may be submitted on claims to Medicare, Medicaid, and most commercial payers. Meanwhile, the CDC/NCHS ICD-10-CM tool confirms its active status. It also shows the parent-code hierarchy coders need when checking that they have reached the most specific option.

Code details at a glance

Clinical description: lateral collateral ligament sprain of the left knee

By definition, the lateral collateral ligament (LCL), also called the fibular collateral ligament, runs from the lateral femoral condyle to the fibular head. Specifically, it sits on the outer side of the knee, and its job is resisting varus stress. In other words, that means inward-directed force pushing the knee into a bow-legged position.

Generally, LCL injuries are less common than MCL sprains. Even so, they still account for a meaningful share of sports-related knee injuries, particularly in contact sports, skiing, and activities with sudden direction changes.

Typically, the mechanism is a direct blow to the medial side of the knee, or a non-contact varus force during a cutting movement. As a result, patients present with lateral knee pain, localized tenderness over the LCL, and a positive varus stress test. In turn, severity grade matters for both clinical management and documentation.

Notably, all three severity grades are coded with S83.422A during the initial treatment encounter. That said, the grade itself is not carried in the ICD-10-CM code. Instead, it lives in the clinical note, where it supports medical necessity for the CPT procedure codes you bill.

In addition, grade drives the rehabilitation timeline, and a staged framework like the return-to-running protocol decides when the patient resumes loading.

Understanding the 7th character: A, D, and S suffixes

Overall, the 7th character is the most common source of encounter-coding errors in musculoskeletal injury cases. Specifically, the ICD-10-CM Official Coding Guidelines (Section I.C.19) define three valid extensions for S83.422, and each one marks a distinct phase of care.

Key distinction. Importantly, initial encounter does not mean the patient’s first visit to any provider. Rather, it means the visit falls inside the active treatment phase.

For example, a patient referred from the ER to a sports medicine physician on day three still gets the A suffix, because active treatment is ongoing. Then, move to D once the plan turns to routine follow-up and healing checks.

S83.422A vs the other collateral ligament sprain codes

As noted, S83.422A sits inside S83.4, the subcategory for sprains of the collateral ligaments of the knee. Consequently, picking the wrong sibling is one of the most common errors in knee injury coding. For reference, the AAPC Codify lookup confirms every variant below as billable with its 7th character attached.

The three specificity levers in S83.4

In short, S83.4 splits on two axes before the 7th character is even added. Specifically, the fifth character names the ligament and the sixth names the side.

• Ligament – S83.42 for the LCL, S83.41 for the MCL, and S83.40 when the note names neither.
• Side – 1 for right, 2 for left, and 9 for unspecified.
• Phase of care – the 7th character A, D, or S.

Interestingly, code lookups for LCL injuries rarely surface S83.40 at all. Even so, that is exactly where a claim lands when the note says lateral knee sprain without naming the ligament. For reference, the table below maps all nine initial-encounter codes in S83.4.

In practice, swap A for D or S to reach the subsequent-encounter and sequela versions of any row. By contrast, fracture categories run a longer extension set, so S49.102G marks delayed healing rather than a phase of care. For that reason, the CDC/NCHS ICD-10-CM code files are the free reference for checking every S83.4 variant against the current release.

ICD-10-CM code hierarchy: where S83.422A sits

Overall, walking the hierarchy is the fastest way to confirm you have reached the most specific valid code. For instance, block S80-S89 holds every knee and lower leg injury, so codes like S80.812A and S81.009S sit alongside S83.422A.

1. Chapter 19 – Injury, poisoning and certain other consequences of external causes (S00-T88)
2. Block S80-S89 – Injuries to the knee and lower leg
3. Category S83 – Dislocation and sprain of joints and ligaments of knee
4. Subcategory S83.4 – Sprain of collateral ligament of knee
5. Subcategory S83.42 – Sprain of lateral collateral ligament of knee
6. Base code S83.422 – Sprain of lateral collateral ligament of left knee
7. Full code S83.422A – Sprain of lateral collateral ligament of left knee, initial encounter

Importantly, only the last line is billable. Therefore, never submit S83, S83.4, S83.42, or the base code S83.422 on their own, because each stops short of a complete code. Instead, always send the full seven-character version with its 7th character attached.

Coding notes and excludes rules for category S83

Notably, category S83 carries instructional notes that govern every code beneath it. Specifically, they tell you which secondary codes the tabular list expects on the claim, and which conditions are simply classified elsewhere.

Specifically, S83 includes avulsion, laceration, sprain, traumatic rupture, traumatic tear, traumatic subluxation, and traumatic hemarthrosis of a knee joint or ligament. A documented LCL tear therefore belongs here rather than under a muscle or tendon code.

• Code also – any associated open wound. For example, an LCL injury with an overlying laceration needs the open-wound code reported alongside S83.422A.
• Excludes2 – seven conditions sit outside category S83. In other words, Excludes2 means the condition is not part of an LCL sprain, so both codes may be reported when the record documents both.

These are the seven conditions on the S83 Excludes2 list:

• Derangement of patella (M22.0-M22.3)
• Injury of patellar ligament or tendon (S76.1-)
• Internal derangement of knee (M23.-)
• Old dislocation of knee (M24.36)
• Pathological dislocation of knee (M24.36)
• Recurrent dislocation of knee (M22.0)
• Strain of muscle, fascia and tendon of lower leg (S86.-)

Importantly, category S83 carries no Excludes1 note at all, which is worth knowing before you drop a code from a claim. For example, a patient with long-standing lateral instability from an old injury still usually belongs on a sequela code or an M-chapter code. In that scenario, sending S83.422A claims an acute injury the record does not support.

Common CPT codes paired with S83.422A

Generally, CPT pairings depend on the setting, the severity, and the treatment approach. Even so, the combinations below are common with S83.422A in orthopedic and physical therapy practice. Either way, verify each one against the current CMS fee schedule and your payer policies before billing.

Typically, physical therapy visits split into evaluation and treatment codes. Specifically, the first evaluation is billed with a code such as 97161, and the complexity level depends on the findings and the decision-making involved.

In practice, practices using sports medicine software can hold these pairings in the workflow at the point of care. In addition, an AAPC CPT-to-ICD-10 crosswalk helps confirm the medical necessity linkage before submission.

Importantly, none of these pairings is automatically approved. Instead, every payer keeps its own medical necessity rules, and coverage for a given CPT code with S83.422A varies between Medicare, Medicaid, and commercial plans. Because of that, check the current CMS Physician Fee Schedule and the payer policy before you bill the surgical codes in particular.

Documentation requirements for S83.422A

Overall, claims using S83.422A need documentation that supports every element of the code. Specifically, CMS guidance and the ICD-10-CM Official Coding Guidelines (Section I.C.19) set clear expectations for injury coding. As a result, practices with structured musculoskeletal notes field fewer payer queries and clear claims faster.

The record has to support all five elements that make S83.422A specific and defensible:

• Laterality – the note must state left knee in the body of the record. However, a checkbox or a pre-populated template field carried over from an earlier visit does not count.
• Ligament specificity – the LCL, or the fibular collateral ligament, has to be named. Otherwise, knee sprain on its own supports only S83.402A.
• Phase of care – the note must show active treatment. For the A suffix, the provider is starting or continuing active care rather than monitoring a healed injury.
• Mechanism of injury – Chapter 19 codes are far easier to defend when the record says how the injury happened. It also supports medical necessity for imaging and therapy.
• Physical findings – lateral joint-line tenderness, varus stress test results, and functional limits belong in the note. In turn, they support the diagnosis and the paired CPT codes.

In practice, templated musculoskeletal exam fields inside clinical documentation tools keep all five in front of the clinician at the point of care. In addition, digital intake forms can capture laterality and mechanism before the clinician enters the room. As a result, that closes the most common shortfall, where a patient says “my knee” and never names the side.

Coding tips and common errors to avoid

Overall, most errors with S83.422A fall into one of five patterns. Fortunately, each is preventable with a structured note template and a pre-bill review. For that reason, practices handling high musculoskeletal claim volumes often lean on dedicated claims management software to surface inconsistencies before submission.

Overall, a standing pre-bill checklist that verifies ligament, side, and phase of care catches most of these errors before submission. In practice, building the check into physical therapy practice management routines beats leaving it to memory. Similarly, structured tools do the same job earlier, which is what the Ottawa Ankle Rules calculator does for imaging decisions.

How Pabau supports accurate ICD-10 coding for musculoskeletal injuries

Typically, orthopedic and sports medicine practices work through high volumes of lateralized injury codes across dozens of body regions. Consequently, the margin is narrow, since one wrong digit in the sixth or seventh position sends the claim down a different adjudication path.

Typically, most teams reconstruct the missing detail at billing, days after the visit, from a note that never captured it. Pabau’s physical therapy EMR software puts documentation and billing in the same record instead. As a result, clinicians record the ligament, the side, and the phase of care while the patient is in the room.

Those documented codes then flow straight onto the claim, so nobody is guessing later. In addition, you can build note templates for common knee presentations that prompt for each field a specific code needs.

Overall, paired with digital intake forms that capture laterality up front, the workflow removes the most frequent cause of unspecified-code claims. Ultimately, the result is fewer payer queries and a shorter path to payment.

Accurate ICD-10 coding starts with the right documentation workflow

Overall, Pabau helps orthopedic and sports medicine practices document laterality and ligament detail at the point of care, then submit those codes on cleaner claims. See how the claims management tools work in practice.

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Conclusion

In short, ICD-10 Code S83.422A is straightforward once the note carries three things. Specifically, the clinician has to name the ligament, the side, and the phase of care. Miss any one of them and the claim drops to a less specific sibling or the wrong encounter type.

Typically, most of the damage shows up as slower payment rather than outright denial. After all, a records request, a resubmission, and a coder rebuilding detail the clinician already knew all cost time. In short, capturing the three elements during the visit is cheaper than reconstructing them at pre-bill review.

Overall, Pabau’s structured note templates and intake forms help practices capture those three elements before the billing team sees the encounter. Book a demo to see how Pabau handles musculoskeletal documentation end to end.

Continue your research

Coding another sprain with a 7th character? For instance, S93.619A walks through the same extension rules for a tarsal ligament sprain.

Assessing lower-limb range of motion? Similarly, Knee to wall test gives you a repeatable ankle dorsiflexion measure for rehabilitation notes.

Ruling out an Achilles rupture? In turn, Thompson test covers the procedure, the interpretation, and the accuracy figures behind the calf squeeze.

Need a scored outcome measure for therapy notes? Likewise, Functional reach test explains how to administer it and how to read the result.

Checking anterior thigh tightness after a knee injury? Finally, Ely’s test shows how to perform it and how to grade what you find.

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