A 58-year-old orthodontist running her own practice pulls in $650,000 a year, maxes her 401(k), and still hands the IRS a six-figure check every April. On a solo practitioner forum this spring, one poster in nearly identical circumstances asked what most high-earning professionals eventually ask: Am I missing something my accountant should have flagged years ago? The answer, for practice owners with W-2 employees they can cover, is almost always yes.
The strategy pairs two qualified plans the IRS has permitted for decades, though fewer than 1 in 20 small-practice owners actually use them together. Together, they can shelter roughly $300,000 of ordinary income in a single tax year, and for a 58-year-old in the top bracket the mechanics are hard to argue with.
Why the Standard 401(k) Alone Leaves Money on the Table
The 2026 employee deferral limit is $24,500, with a $8,000 catch-up for anyone age 50 to 59, for a maximum of $32,500 from the employee side. Add employer profit-sharing on top, and the combined 415(c) ceiling is $72,000.
There is a wrinkle our orthodontist cannot ignore. Under SECURE 2.0, starting in 2026, any employee age 50 or older who earned more than $150,000 in FICA wages the prior year must direct catch-up contributions into a Roth 401(k). That $8,000 no longer reduces her taxable income. It grows tax-free instead.
Even doing everything right, the 401(k) alone tops out near $80,000 of shelter. At her 37% federal marginal rate (which kicks in above $640,600 for single filers in 2026), that saves roughly $27,000. Useful, but the ceiling is the ceiling.
The Cash Balance Plan: Where the Other $220,000 Comes From
The overlooked piece is a cash balance plan, a hybrid defined benefit vehicle stacked on top of the 401(k). Because contribution limits are actuarially driven by age and expected retirement income rather than by a flat cap, a 58-year-old owner can typically shelter an additional $200,000 to $260,000 per year, depending on compensation and staff demographics. Add the Solo 401(k) side, and total pretax shelter clears $300,000.
Sheltering $300,000 at a 37% marginal rate saves $111,000 in federal income tax alone. Over a 7-year run to age 65, that is close to $1 million in avoided current taxes, compounding inside a plan yielding whatever the underlying portfolio earns. With the 10-year Treasury at 4.69%, a balanced allocation is not fantasy math.
Cash balance plans require nondiscrimination testing, meaning the practice must fund meaningful contributions for eligible staff, generally 5% to 7.5% of their compensation. For a small orthodontic office with three or four W-2 employees, the staff cost is often a fraction of what the owner shelters.
Layer a Mega Backdoor Roth on Top
If the 401(k) document permits after-tax contributions and in-plan Roth conversions, the owner can also fill the gap between her elective deferrals plus profit-sharing and the $72,000 combined limit with after-tax dollars, then convert them. Growth becomes tax-free rather than tax-deferred. Roth 401(k) balances have no lifetime required distributions, per rule changes effective since 2024, which matters when RMDs at 75 threaten to push a retiree back into a 32% bracket.
Three Moves Before Year-End
- Get a cash balance illustration. Any third-party administrator serving medical or dental practices can model contribution capacity based on W-2, age, and staff census. Ask for the exact owner allocation and the required staff contribution side by side.
- Confirm your plan document allows after-tax contributions and in-plan Roth conversions. Many off-the-shelf Solo 401(k) documents do not. Amending the plan or switching providers is a one-time cost that unlocks the mega backdoor mechanic permanently.
- Route the $8,000 catch-up to the Roth side and verify payroll coding. If your W-2 for 2025 shows Social Security wages in Box 3 above $150,000, the pretax catch-up is no longer an option in 2026, and a plan without a Roth feature blocks catch-ups entirely.
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